(concurring in part/dissenting in part) — The majority correctly recognizes the decision denying petitioner Gary Horsley his constitutionally mandated day in court was error, requiring reversal and new trial. Majority at 511. However the majority persists in its refusal to honor Horsley’s right to amend his original pro se answer, claiming respondent Diana Wilson would somehow be prejudiced if she were forced to respond to such an amended answer at a trial some five years or more after the amendments were proposed.6 Majority at 511-12. Balderdash.
Motion to Amend
Since the American Civil War we have stated failure to grant leave to amend where the interests of justice would be promoted is an abuse of discretion. See Newberg v. Farmer, 1 Wash. Terr. 182, 183-84 (1862). As the majority notes, abuse of discretion is “ ‘discretion manifestly unreasonable, or exercised on untenable grounds, or for untenable reasons.’ ” Majority at 505 (quoting State ex rel. Carroll v. Junker, 79 Wn.2d 12, 26, 482 P.2d 775 (1971)). Such *513determination “depends upon the comparative and compelling public or private interests of those affected by the order or decision and the comparative weight of the reasons for and against the decision . . . id. at 26, requiring the trial court’s alleged abuse be examined in light of the purpose of our rule governing amendments.
Horsley was entitled to amend his answer with the leave of the court, and the rule requires that such leave “shall be freely given when justice so requires.” CR 15(a). Shall means shall. It imposes “a mandatory duty.” See, e.g., Waste Management of Seattle, Inc. v. Utilities & Transp. Comm’n, 123 Wn.2d 621, 629, 869 P.2d 1034 (1994). This ideal of freely granting the right to amend is well integrated into our jurisprudence, and, as we have articulated, amendments “have always been . . . liberally allowed.” J.D. O'Malley & Co. v. Lewis, 176 Wash. 194, 198, 28 P.2d 283 (1934) (emphasis added).
“The purpose of pleadings is to ‘facilitate a proper decision on the merits’, and not to erect formal and burdensome impediments to the litigation process.” Caruso v. Local Union No. 690, 100 Wn.2d 343, 349, 670 P.2d 240 (1983) (quoting Conley v. Gibson, 355 U.S. 41, 48, 78 S. Ct. 99, 2 L. Ed. 2d 80 (1957)). When construing the comparable federal rule, Fed. R. Civ. E 15(a),7 the United States Supreme Court has said the declaration that leave shall be freely granted constitutes a “mandate [that] is to be heeded.” Foman v. Davis, 371 U.S. 178, 182, 83 S. Ct. 227, 230, 9 L. Ed. 2d 222 (1962) (citation omitted). Failure to grant leave without proper justification is therefore not an exercise of discretion, but an abuse of discretion. Id.
The touchstone for denial is the prejudice the amendment would cause the nonmoving party. Caruso, 100 Wn.2d at 350. The party opposing the amendment has the burden to show it. See id. at 351; In re Revocation of License of *514Campbell, 19 Wn.2d 300, 307, 142 P.2d 492 (1943); Tagliani v. Colwell, 10 Wn. App. 227, 234, 517 P.2d 207 (1973). Plaintiffs are allowed leave to amend “ ‘unless it appears to a certainty that plaintiff would not be entitled to any relief under any state of facts . . . .’ ” Adams v. Allstate Ins. Co., 58 Wn.2d 659, 672, 364 P.2d 804 (1961) (quoting Fuhrer v. Fuhrer, 292 F.2d 140, 143 (7th Cir. 1961)). Therefore the question is: Has Wilson shown she would be prejudiced if Horsley was granted leave to amend?
This search for prejudice may be enlightened by a number of factors, including undue delay, unfair surprise, jury confusion, introduction of remote issues, or lengthy trial. Herron v. Tribune Publ’g Co., 108 Wn.2d 162, 165-66, 736 P.2d 249 (1987). However Wilson has simply not shown prejudice based upon these or any other factors.
Delay, per se, is no reason for denial. If it were, no leave to amend would ever be allowed as amendments are by their nature delayed beyond the original pleading. Delay, excusable or not, is not sufficient reason to deny a motion to amend unless it works some undue hardship or prejudice upon the opposing party. Id. Horsley filed his motion to amend seven months after he filed his original pro se answer. How “undue” was this delay is a matter of opinion; however, we have held that a delay of over five years is acceptable absent a showing of prejudice by the party opposing amendment. Caruso, 100 Wn.2d at 349.
The trial court also complained Horsley was seeking to amend “on the eve of trial.” Report of Proceedings (RP) at 8 (May 6, 1994). But Horsley originally sought to amend his answer more than a month and a half before the originally scheduled trial start date, and a hearing was held on the motion a month before trial. In fact the trial actually took place a year after Horsley first sought leave to amend. And the retrial the majority orders today will be substantially after this opinion is published—a minimum of five years after the motion to amend was first made. Yet our rule has been to allow amendment at any stage of the proceeding. Hendricks v. Hendricks, 35 Wn.2d 139, 148, 211 P.2d 715 (1949).
*515When the amendment seeks only to assert a new legal theory based upon the same circumstances set forth in the original pleading, it should be allowed. Herron, 108 Wn.2d at 166. Such is exactly the case here. Horsley’s original answer claimed Wilson, while intoxicated, hurt herself when she struck him. In his amended answer he proposed several alternative theories, including comparative negligence, self-defense, and Wilson’s lack of mitigation, based upon the same facts underlying his original answer. Such is exactly the situation which those policies underlying CR 15 are designed to promote, dictating leave to amend should be freely given. See Herron, 108 Wn.2d at 166-67. However this is exactly the opposite from the conclusion reached by the trial judge who denied leave to amend precisely because Horsley raised new theories based on facts known at the time of his original complaint. RP at 8-9.8
The majority gives short shrift to the mandatory language of CR 15 and the numerous cases from this court and the United States Supreme Court which apply it. The bulk of the majority’s analysis is premised upon the fact that Hors-ley sought to amend after arbitration. Majority at 506-07.
But even following arbitration an aggrieved party is entitled by right to a trial de novo in superior court “on all issues of law and fact.” RCW 7.06.050; see also Mandatory Arbitration Rule (MAR) 7.1. That is, Horsley was entitled to “[a] new trial ... in which the whole case is retried as if no trial whatever had been had in the first instance.” Black’s Law Dictionary 1505-06 (6th ed. 1990). See also In re Littlefield, 61 Wash. 150, 153 112 E 234 (1910) (trial de novo means trial anew); MAR 7.2(b)(1) (“The trial de novo shall be conducted as though no arbitration proceeding had occurred.”). The Washington State Bar Association in 1989 *516proposed amendments to the MARs that “ ‘were necessary to ensure the effectiveness of [the de novo review] mandate.’ ” 4A Lewis H. Orland & Karl B. Tegland, Washington Practice: Rules Practice 41 (quoting Washington State Bar Association comments on MAR 7.2). Where there is no ambiguity in the language there is no room for construction. POWER v. Utilities & Transp. Comm’n, 101 Wn.2d 425, 429, 679 P.2d 922 (1984).
In almost exact contradiction to the rule our majority opines the trial de novo must be conducted exactly in accord with the arbitration proceeding which previously occurred, holding the trial court may “consider” what occurred at the arbitration and base its decisions accordingly. This view is supported by a single sentence in a practitioner prepared deskbook, majority at 506 (quoting M. Wayne Blair, Mandatory Arbitration in Washington, in Washington State Bar Ass’n, Alternate Dispute Resolution Deskbook: Arbitration and Mediation in Washington § 2.3(7)(b), at 2-60 (2d ed. 1995) (ADR Deskbook), but is contrary to the express language of MAR 7.2(b)(1) that “[t]he trial de novo shall be conducted as though no arbitration proceeding had occurred.”
Even the deskbook observes that the decision to amend is governed by CR 15. ADR Deskbook § 3(7)(b). But CR 15 is exactly what both the trial court and the majority ignore by failing to articulate exactly how Horsley’s proposed amendments would prejudice Wilson. The majority states that the. proposed amendments would “substantially change the case.” Majority at 507. They don’t, but even if they did, that, more or less, is what amendments are for. Here the amendments and counterclaims are based on the original facts. All the more reason they should be granted.9 See Herron, 108 Wn.2d at 166-67. Naked claims of prejudice do not establish it.
*517Finally, the majority claims reduction of litigation as its purpose.10 Majority at 507. I would rather posit our rules and holdings require our focus on a single value: the interest of justice. See CR 15(a); Caruso, 100 Wn.2d at 349; Adams, 58 Wn.2d at 671-72; see also Foman, 371 U.S. at 182. The Court of Appeals has held an abuse of discretion occurs where a trial court refused to allow an amendment after an oral ruling on a summary judgment motion. Tagli-ani, 10 Wn. App. at 234. Similarly, the United States Supreme Court applied the identical Fed. R. Civ. E 15(a) to reverse a trial court’s denial of leave to amend when the motion was made after the petitioner’s complaint had been dismissed for failure to state a claim. Foman, 371 U.S. at 179. And our territorial court went so far as to hold that amendment must be allowed, if justice requires, on an appeal from the Justice’s Court to the District Court. New-berg, 1 Wash. Terr, at 183-84. The considerations of justice which prompted these historical results do not change over time.
Here Horsley was unjustly forced to proceed to trial within the scope of his original pro se answer, a single handwritten paragraph prepared without legal assistance.11 There is just reason, and plenty of time, to allow the amendment on remand.
*518Johnson and Madsen, JJ, concur with Sanders, J.
Horsley filed his motion to amend on April 18, 1994. Clerk’s Papers (CP) at 13.
Rule 15 of Superior Court Civil Rules was taken from Fed. R. Civ. E 15 and was designed to facilitate the same ends as Fed. R. Crv. P. 15. Caruso, 100 Wn.2d at 349. See also Adams v. Allstate Ins. Co., 58 Wn.2d 659, 672, 364 P.2d 804 (1961) (“Our rule [15(a)] is the exact counterpart of the provision in the Federal rules of civil procedure . . . .”).
Horsley also added a counterclaim in his amended answer which the trial judge held was “compulsory” and therefore should have been pleaded prior to arbitration. CP at 30 (citing CR 13(a) which governs claims that arise out of the original transaction or occurrence). But the rules clearly intend that the amended complaint can set forth additional claims arising out of the original conduct, as CR 15(c) covers the relation back of such claims. Why would a rule exist to govern such claims if they are not to be permitted? See Herron, 108 Wn.2d at 166.
Of course, should a party prevail on a counterclaim raised after arbitration, a question may arise regarding the calculation and/or propriety of reasonable attorney fees allowed by MAR 7.3, which requires the superior court to assess fees against the party who fails to improve its position at a trial de novo. However, the solution is to assess or fashion fees in consideration of the effect of the new claim, *517not to deny a party its statutory right to a trial de novo. See Christie-Lambert Van & Storage Co. v. McLeod, 39 Wn. App. 298, 305, 693 P.2d 161 (1984).
The “advantages” of mandatoiy arbitration over the traditional method of securing rights through access to the courts is, itself, a subject of extensive comment. See, e.g., Judge G. Thomas Eisele, Differing Visions—Differing Values: A Comment on Judge Parker’s Reformation Model for Federal District Courts, 46 SMU L. Rev. 1935, 1959 (1993) (“[Cjourt-annexed arbitration is surely giving traditional arbitration a bad name. This is because the objectives of the two procedures are different. Traditional arbitration may go to some lengths to establish the true facts, but that would defeat the purpose of court-annexed arbitration. If court-annexed ADRs [alternative dispute resolutions] are not limited in time and procedure, they may last as long as a trials. Therefore, if one’s only justification for such diversions in the first place is the alleged reduction in costs and delays, we have a problem, don’t we?”).
The entire original answer is set out by the majority at 502.