concurring:
¶ 53 I agree with the lead opinion’s conclusion that H.B. 213 does not effect an unconstitutional taking of plaintiffs’ property. I write separately to express my opinion that state employees had no vested right to exchange their accrued sick leave for health insurance because the plain language of the 2004 version of the Unused Sick Leave Retirement Option Program (“Option Program”), Utah Code Ann. § 67-19-14.2, limits ed participation in the program to those employees eligible to receive retirement benefits. Accordingly, state agencies could offer the program only to those employees who had elected to retire. Further, were I to assume the existence of a statutory ambiguity with regard to the vesting issue, I would conclude that extrinsic evidence of legislative intent and relevant principles of statutory construction dictate the same result.
¶ 54 Plaintiffs raise only one claim on appeal, a facial challenge to the constitutionality of H.B. 213. Resolution of their facial challenge hinges on whether they can establish that they had a vested right to exchange 100% of their accrued sick leave for insurance. If so, H.B. 213 effects an unconstitutional taking of their property. If not, their challenge fails.
¶ 55 As the lead opinion correctly states, because the terms of public employment are generally governed by statute and legislative policy, they are subject to legislative change. We have recognized only two exceptions pursuant to which a public employee may acquire a contractual interest in employment benefits. The first is when a governmental employer has entered into an express or implied contract by voluntarily undertaking obligations beyond those provided by statute. See Canfield v. Layton City, 2005 UT 60, ¶ 16, 122 P.3d 622. The second is when an employee acquires a vested contractual interest pursuant to the terms of the operative statute. See Hansen v. Pub. Employees Ret Sys., 122 Utah 44, 246 P.2d 591, 595-96 (1952). I will address each exception in turn.
I. PLAINTIFFS CANNOT ESTABLISH ANY PROPERTY INTEREST PURSUANT TO THE VOLUNTARY UNDERTAKING EXCEPTION
¶ 56 To give rise to a protectable property interest under the voluntary undertaking exception, the voluntary undertaking must stem from an agreement that alters or adds to the statutory terms and conditions of public employment. See Buckner v. Kennard, 2004 UT 78, ¶ 34, 99 P.3d 842. This exception appears inapplicable in a case such as this where plaintiffs have raised only a facial challenge to the governing statute.
¶ 57 As the trial court found, the plaintiffs in this case rested their claims entirely on *221the statutory language. Plaintiffs presented no evidence and the trial court made no finding of any obligation or undertaking by state agencies apart from the decision of the agencies to participate in the Option Program as set forth in the governing statute. While agency participation in the Option Program was voluntary, the terms of that participation were indisputably statutory. Because plaintiffs lodge only a facial challenge to H.B. 213, their voluntary undertaking argument is entirely dependent upon the statutory language, which, as discussed below, dictates that any offer of program benefits may occur only upon retirement.
II. THE PLAIN STATUTORY LANGUAGE DOES NOT CREATE ANY VESTED RIGHTS IN STATE EMPLOYEES UNTIL THEY ELECT TO RETIRE
¶ 58 I believe that the statutory language is determinative in resolving plaintiffs’ claim that they had a vested right to exchange 100% of their unused sick leave for health insurance. In prior cases involving the claims of public employees, we have reasoned that the rights at issue “rest largely upon the language of the particular statute” involved. Driggs v. Utah Teachers Ret. Bd., 105 Utah 417, 142 P.2d 657, 663 (1943); see also Newcomb v. Ogden City Pub. Sch. Teachers’ Ret. Comm’n, 121 Utah 503, 243 P.2d 941, 944 (1952) (“[T]he rights of pensioners must be determined by the purpose and language of the retirement act.”).
¶ 59 When construing statutory language, this court adheres to the well-accepted rule that we do “ ‘not look beyond the plain language of [the] provision unless we find some ambiguity in it.’ ” State v. Ostler, 2001 UT 68, ¶ 7, 31 P.3d 528 (alteration in original) (quoting In re Worthen, 926 P.2d 853, 866 (Utah 1996)); Wilcox v. CSX Corp., 2003 UT 21, ¶ 8, 70 P.3d 85. Only upon finding ambiguity may we “seek guidance from the legislative history and relevant policy considerations.” Ostler, 2001 UT 68, ¶ 7, 31 P.3d 528 (internal quotation marks omitted). In other words, we may not resort to extrinsic aids of interpretation unless we first find ambiguity in the statutory text. See id.
¶ 60 Like a contract, a statute is ambiguous when it may reasonably “be understood to have two or more plausible meanings.” Alf v. State Farm Fire & Cas. Co., 850 P.2d 1272, 1274 (Utah 1993) (internal quotation marks omitted) (defining ambiguity in the context of a contract); Saleh v. Farmers Ins. Exch., 2006 UT 20, ¶ 15, 133 P.3d 428. But determining whether there are two or more plausible meanings depends not only on the text of the particular provision at issue, but also on the text of the statute as a whole. See Morton Int’l v. Auditing Div. of the Utah State Tax Comm’n, 814 P.2d 581, 591 (Utah 1991). Indeed, “[w]e ‘read the plain language of the statute as a whole, and interpret its provisions in harmony with other statutes in the same chapter and related chapters.’ ” State v. Barrett, 2005 UT 88, ¶ 29, 127 P.3d 682 (quoting Miller v. Weaver, 2003 UT 12, ¶ 17, 66 P.3d 592). As a result, a statute susceptible to competing interpretations may nevertheless be unambiguous if the text of the act as a whole, in light of related statutory provisions, makes all but one of those meanings implausible. Id. When viewing the act as a whole does not eliminate duplicative yet plausible meanings, the statute is ambiguous, and we may resort to extrinsic interpretive tools to resolve the ambiguity. See Ostler, 2001 UT 68, ¶ 7, 31 P.3d 528.
¶ 61 As the lead opinion ably explains, the statutory iterations of the Option Program from 1983 through 2004 all required that employees convert at least 25% of their accrued sick leave to cash or a 401(k) contribution. In all respects material to this appeal, these prior iterations are therefore indistinguishable from H.B. 213. As a result, only the 2004 iteration of the Option Program could have conceivably vested in state employees the right to convert to health insurance the first 25% of their accrued sick leave. I therefore confine my analysis to the 2004 statute.
¶ 62 The 2004 statute provides that “[a]n agency may offer the [Option Program] to an employee who is eligible to receive retirement benefits in accordance with Title 49, Utah State Retirement and Insurance Benefit Act.” Utah Code Ann. § 67 — 19—14.2(l)(b) (2004). The lead opinion concludes that this language is ambiguous because it could be *222construed to apply to (1) all regular, full-time state employees; (2) all state employees who have reached the requisite age and years of employment necessary to retire under a particular retirement system; or (3) retiring employees. I disagree and conclude that the statutory language effectively precludes agencies from even offering the Option Program until an employee has elected to retire.
¶ 63 My analysis necessarily starts with “the usual and natural meaning” of the operative terms of the statute. See Saleh, 2006 UT 1, ¶ 17 (internal quotation marks omitted). “Offer” is a verb meaning “an instance of presenting something for acceptance.” Black’s Law Dictionary 1113 (8th ed.2004). “Eligible” is an adjective referring to one who is “legally qualified for a[ ] ... privilege, or status,” or “[f]it and proper to ... receive a benefit.” Id. at 559. “Receive” is a verb meaning “to come into possession of.” Merriam-Webster’s Collegiate Dictionary 975 (10th ed.1998). And because the statute is phrased in the present tense, it allows an agency to offer the “Program to an employee who is eligible to x-eceive retirement benefits,” not to an employee who may become or who could be eligible to receive benefits some time in the future. Utah Code Ann. § 67-19 — 14.2(l)(b) (emphasis added). Moreover, subsection (2) of the statute specifically states that an employee may purchase continuing medical and life insurance benefits “upon retirement.” Id. § 67-19-14.2(2).
¶ 64 The commonly understood definitions of these operative terms dictate that state agencies could present the Option Program for acceptance only to those employees who were currently legally qualified to come into possession of retirement benefits. This renders implausible the interpretation urged by plaintiffs, that the language allowed state agencies to offer the Option Program to all l’egular, full-time employees. It also undercuts the suggestion that the statute could plausibly be interpreted to allow agencies to offer the Option Program to employees with the i’equisite age and years of service necessary to enable them to retire but who have not elected to do so. Indeed, it is implausible to suggest that an employee who has not yet elected to retire could be eligible to receive (as opposed to be eligible to apply for) benefits. I therefore conclude that the text of section 67-19-14.2 prevents an agency from offering the Option Program until an employee elects to retire.
¶ 65 The majoxdty rejects this interpretation as implausible because it believes such an interpretation is inconsistent with the stated legislative purpose of the program, which is to induce employees to reduce sick leave abuse. I find no such inconsistency. The fact that the program did not vest employees with the contractual right to exchange 100% of their accrued sick leave for health insurance does not necessarily mean that it could not operate as an effective incentive program.
¶ 66 Individuals routinely assume some degree of risk in making life decisions, and there is no reason why employees would necessarily require an ironclad guarantee before being motivated to reduce sick leave abuse. Nearly all aspects of compensation and retirement benefits in both the public and private sectors are offered as inducements to accept employment. But that does not mean that those benefits become immutable contractual guarantees as soon as an employee begins work or that any employee can reasonably expect that an employer will be offering the same benefits decades into the future. This principle is aptly illustrated by the fact that the pi’e-2004 iterations of the Option Program did not even include the option of exchanging the first 25% of accrued sick leave for health insurance. It is also illustrated by the majority’s tacit assumption that the statutory purpose of reducing sick leave abuse is fulfilled by its construction of the statute — a construction that in practical effect differs not at all from the interpretation urged by the State and adopted by me.
¶ 67 My plain language construction of the Option Program is bolstered by the language of title 49 of the Utah Code, which is referenced in the 2004 statute. Title 49 of the Utah Code consists of the Utah State Retirement and Insurance Benefit Act. Chapter 11 of that act governs the administration of the system, while chapters 12 through 21 establish various retirement systems for specific classes of public employees. Although some *223of the specific provisions of these various systems differ, the statutory structure of each follows a similar pattern, and each is consistent with my conclusion that eligibility to receive retirement benefits can occur only when an employee elects to retire.
¶ 68 First, each of the various systems distinguishes those employees who are merely eligible to participate in the retirement system from those employees who are eligible to receive an allowance from the system. Membership in the system is available to all “regular full-time employee[s] of a participating employer” and begins on “the effective date of employment.” Utah Code Ann. § 49-12-201 (2002); see, e.g., id. §§ 49-13-201, 49-14^-201 (Supp.2005). When the legislature referred to the class of employees eligible for participation in one of the state retirement systems, it used the phrase “eligible for service credit.” E.G., Utah Code Ann. § 49-12-201. Because eligibility for service credit differs from eligibility to receive retirement benefits, I reject plaintiffs’ contention that the phrase “eligible to receive retirement benefits” could plausibly be interpreted to refer to all regular, full-time state employees.
¶ 69 I find the provisions of title 49 addressing eligibility to receive allowances particularly helpful. For example, Utah Code section 49-12-401 (2002) provides:
(1) A member is qualified to receive an allowance from this system when:
(a) the member ceases actual work for a participating employer in this system before the member’s retirement date and provides evidence of the termination;
(b) the member has submitted to the office a notarized retirement application form that states the member’s proposed retirement date; and
(c) one of the following conditions is met as of the member’s retirement date:
(i) the member has accrued at least four years of service credit and has attained an age of 65;
(ii) the member has accrued at least ten years of service credit and has attained an age of 62 years;
(in) the member has accrued at least 20 years of service credit and has attained an age of 60 years; or
(iv) the member has accrued at least 30 years of service credit.
Each of the various retirement acts comprising title 49 contains a similar provision. See, e.g., id. §§ 49-13-401, 49-14-401 (Supp. 2005), 49-15-401 (2002).
¶ 70 At first glance, because these provisions use the phrase “qualified to receive an allowance,” they do not appear to clarify the meaning of the phrase “eligible to receive retirement benefits” as used in section 67-19-14.2. But further examination of title 49 demonstrates that the legislature used the terms “allowance” and “retirement benefits” interchangeably. See, e.g., id. ■§§ 49-12-402(2) (Supp.2005) (the “Option One benefit is an annual allowance”), 49-12-405, (4) (referring to Option Three allowance as “[s]er-vice retirement benefits” and stating that “benefits payable under this section are retirement benefits”), 49-ll-401(3)(b) (“[a]n allowance or other benefit”), 49-11-405(3) (2002) (referring to a “member’s allowance” as “benefit”). Moreover, the notion of being qualified or eligible to actually “receive” an allowance or benefit is common to both section 67-19-14.2 and section 49-12-401. They therefore support my conclusion that those employees who are “eligible to receive retirement benefits” must have satisfied the conditions specified by section 49-12-401 for receipt of an allowance.
¶71 Justice Nehring opines that my conclusion in this regard is undercut by other provisions of title 49. In fact, however, much of his analysis improperly draws on sources extrinsic to the statutory language. He suggests that the phrase “eligible to receive retirement benefits” is distinct from the phrase “qualified to receive an allowance” by drawing a distinction between the terms “eligible” and “qualified.”1 But this distinction stems from an inspection of the statutory *224language of pre-2004 versions of section 67-19-14. Prior to 2004, the statute provided that “[a]n employee must be eligible for retirement benefits to qualify for the program.” Utah Code Ann. § 67-19-14 (Supp.1999). Because the predecessor versions of the 2004 statute used both “eligible” and “qualify,” Justice Nehring concludes that these words cannot be ascribed the same meaning. He then suggests that, because legislative history indicates that the legislature did not wish to make substantive changes when passing the 2004 version of the statute, the reference to title 49 must have implicitly preserved the distinction found in prior iterations of the program. Finally, Justice Nehring construes “eligible to receive retirement benefits” to mean “eligible to retire.” I pause to briefly respond to these contentions.
¶ 72 First, we may not look to extrinsic aids to ascertain the meaning of a statute where the statute is plain on its face. Past versions of a statute and legislative history are indisputably extrinsic aids. As such, they may be helpful in resolving an ambiguity, but they may not be employed to create one. As discussed above, the plain meaning of the statute is that an agency may not offer the Option Program to an employee until she elects to retire. Where the statutory meaning is plain, resort to past versions of the statute and legislative history is improper. Moreover, because the 2004 version of the statute does not use both “eligible” and “qualify,” the dichotomy that Justice Nehring attempts to create between the two terms does not logically become an issue.
¶ 73 In fact, the words “eligible” and “qualified” are synonymous, and our case law does not mandate a distinction between the two in the context of the Option Program. We have stated that we give effect to each term of a statute and assume that the legislature used each term advisedly. State v. Barrett, 2005 UT 88, ¶ 29, 127 P.3d 682. But this does not mean that we must, in all cases, ascribe competing meanings to synonymous terms found in the same statute. This is particularly true here where title 49 uses the words “qualified,” “eligible,” and their variants interchangeably. Compare Utah Code Ann. § 49 — 11—401(3)(c) (Supp.2005) (instructing the board to regulate how service credits should “be credited toward qualification for retirement” (emphasis added)), and id. § 49-ll-403(l)(c) (providing that an employee may purchase service credits based on out-of-state public employment where she “does not qualify for any retirement benefits based on the employment” (emphasis added)), with id. § 49-ll-404(2)(d) (2002) (basing cost-of-living increase factor “on the date the member is eligible to receive benefits under a benefit protection contract” (emphasis added)). Therefore, while Justice Nehring concludes that the legislature used the word “qualify” rather than the phrase “be eligible” in pre-2004 iterations of the statute for the sole purpose of signaling that “eligible” means less than “legally qualified,” I cannot make that inferential leap.
¶ 74 More fundamentally, Justice Nehring defines “eligible” as “able to choose.” But the prevailing definition of “eligible” is “qualified to be chosen.” Merriam-Webster’s Collegiate Dictionary 374 (10th ed.1998). If we insert this definition into the 2004 statute, it reads, “An agency may offer the [Option Program] to an employee who is [qualified to be chosen] to receive retirement benefits.” Because the State may not require an employee to receive retirement benefits before the employee fulfills all the requirements of Utah Code section 49-12-401, including filing an application to retire, Justice Nehring’s interpretation is unavailing. In other words, the State may not require that an employee actually receive retirement benefits before the employee has applied for retirement.
¶75 In addition to relying on prior versions of the statute, Justice Nehring relies on legislative history for the proposition that the 2004 amendment was not intended to make any substantive changes to the Option Program. But in the absence of ambiguity, reliance on legislative history is improper. And if the legislative history of the 2004 statute truly indicates no intent to make substantive changes to the Option Program, it is patently unreliable because plaintiffs’ only claim arises as a result of a substantive change *225wrought by the 2004 amendment2 Thus, Justice Nehring’s conclusion that the phrase “qualify for the program” is functionally equivalent to the phrase “in accordance with Title 49” in the 2004 statute has no basis. Indeed, it is not unreasonable to conclude that the reference to title 49 in the 2004 statute was intended to codify the pre-2004 regulation providing that “the decision to participate [in the Option Program] shall be made at retirement.” Utah Admin. Code r.477-7-6(b) (2000).
¶76 Justice Nehring also relies on other provisions of title 49. But none of these provisions establish the meaning of the phrase “eligible to receive retirement benefits.” For example, Justice Nehring points to the definition of “retirement” contained in Utah Code section 49-11-102(34) (Supp. 2005). It defines “retirement” as the status of an individual who has “become eligible, applies for, and is entitled to receive an allowance.” Because eligibility to receive an allowance is only one of three components of “retirement,” Justice Nehring concludes that eligibility to receive retirement benefits must refer to a status distinct from retirement. I disagree because the other two components of retirement (becoming eligible to apply for retirement and filing a retirement application) are conditions that must necessarily be fulfilled before one is eligible to receive an allowance. Thus, the definition of “retirement” may, in fact, be coextensive with the final act necessary to effectuate retirement— eligibility to receive retirement benefits.3
¶ 77 In summary, I believe that the only way to read section 67-19-14.2 consistently with title 49 is to give “retirement benefits” the same meaning as “allowance.” Therefore, section 67-19-14.2 grants authority to an agency to offer the Option Program to only those employees who have (1) ceased actual work, (2) submitted an application to retire, and (3) attained the requisite age and accumulated the requisite number of service credits. Given this construction of the statute, no public employee could have obtained a vested right in the Option Program prior to submitting an application to retire. In the absence of any vested rights, H.B. 213 cannot effect a taking.
III. ASSUMING AN AMBIGUITY IN THE STATUTE, APPLICABLE CANONS OF STATUTORY CONSTRUCTION DICTATE THE SAME RESULT
¶ 78 Were I to assume the ambiguity of the statute for argument’s sake, I would nevertheless reject plaintiffs’ contention that the statute endowed employees with a vested right to exchange 100% of their accrued sick leave for health insurance. I reach this conclusion on the basis of the statutory history of the Option Program, the implementing regulations, and what I believe to be the applicable canon of statutory construction.
¶ 79 There is a “well-established presumption” that “absent some clear indication that the legislature intends to bind itself contractually, ... ‘a law is not intended to create private contractual or vested rights but merely declares a policy to be pursued until the legislature shall ordain otherwise.’ ” Nat’l R.R. Passenger Corp. v. Atchison, Topeka & Santa Fe Ry. Co., 470 U.S. 451, 465-66, 105 S.Ct. 1441, 84 L.Ed.2d 432 (1985) (citation omitted). In other words, when it is unclear whether a legislature intends to bind itself, a court should not infer such an intent. Cf. id. Because section 67-19-14.2 lacks a *226“clear indication” that “the legislature intended]” to bestow on state employees an irrevocable and unalterable property right to exchange 100% of their banked sick leave for health insurance, I would decline to find such a right.
¶ 80 This conclusion is not only mandated by the applicable canon of statutory construction, it is consistent with the legislative intent. As described in the lead opinion, the various changes in the Option Program from 1979 to 2004 demonstrate that the legislature intended to reserve its ability to modify the menu of available benefits. The changes in the program since its inception are, in fact, persuasive evidence that the legislature did not intend to bind the State to forever redeem 100% of accrued sick leave for health insurance benefits.
¶ 81 This conclusion is also consistent with the interpretation of the statute adopted by the Director of Human Resource Management in the applicable regulations. We previously have recognized that an agency’s interpretation of a statute it administers may be given some weight. See McKnight v. State Land Bd., 14 Utah 2d 238, 381 P.2d 726, 731 (1963). In this case, the director has effectively interpreted the provision at issue by promulgating rules that explicitly allow an agency to offer the Option Program to an employee only “[u]pon retirement.” Utah Admin. Code r.477-7-6 (2005). And the regulations also provide that state agencies may opt in and out of the Option Program on an annual basis, a provision that is wholly inconsistent with the claim of a vested contractual right at any point prior to an employee’s election to retire. See id. Inasmuch as the regulatory interpretation is abundantly reasonable and consistent with the statutory language, I believe it should be given considerable weight.
CONCLUSION
¶82 Although I agree with the result reached by the majority, my reasoning differs. I conclude that both a plain meaning analysis and an analysis that assumes ambiguity lead to the conclusion that the 2004 statute empowered agencies to offer the Option Program to employees only upon retirement. Because the offer to participate in the Option Program could not be made until an employee elected to retire, plaintiffs have no vested contractual right to exchange 100% of their accrued sick leave for health insurance.
. Plaintiffs have never urged the interpretation of the 2004 statute articulated by Justice Nehring, a fact somewhat at odds with Justice Nehring's suggestion that his interpretation is driven by the "plain” language of the statute.
. In actuality, the legislative history indicates that the legislature’s intent was to conform the statute to agency practice. Audio recording: Senate Debate of H. B. 11, 55th Leg. Gen. Sess. (Feb. 26, 2004) available at http://le.utah.gov/asp/audio/in-dex.asp?Sess=2004GS & Bill=HB0011 & Da y=0 & House=S ("[The 2004 amendment] codifies existing procedure ... and the existing way that we use unused sick leave.”).
. Justice Nehring also invokes sections 49-12-701 and 49-11-103(2). Neither applies. While section 49-11 — 103(2) suggests that title 49 should be liberally construed to provide maximum benefits, it does not go so far as to suggest that it may be invoked to create an ambiguity or to countenance a construction inconsistent with the statutory language. It is also questionable whether it even applies at all to our attempt to construe the provisions the 2004 statute, which is found in a different title of the Utah Code.
Section 49-12-701 is equally inapplicable. That section governed an early retirement program that has not been available since 1988. Its value in aiding our interpretation of the 2004 statute is therefore suspect.