Clark v. Commonwealth, 3 Va. App. 474, 480, 351 S.E.2d 42, 45 (1986). In Meadows v. McClaugherty, 167 Va. 41, 187 S.E. 475 (1936), the trial court was reversed because it permitted the defendant to relate the contents of a conversation he previously had with his brothers who were to testify subsequent to the defendant.

When a claim is made under an unambiguous written instrument, however, a signatory to the instrument may introduce parol evidence to establish a defense based on such doctrines as partial integration, collateral contract, fraudulent procurement, mutual mistake, or condition precedent. Id. at 182-83; 320 S.E.2d at 343-44; J.E. Robert Co. v. J. Robert Co., 231 Va. 338, 343, 343 S.E.2d 350, 353 (1986); Walker LaBerge Co. v. First Nat'l Bank of Boston, 206 Va. 683, 688, 146 S.E.2d 239, 244 (1966); Meadows v. McClaugherty, 167 Va. 41, 45, 187 S.E. 475, 477 (1936); Whitaker Fowle v. Lane, 128 Va. 317, 345-46, 104 S.E. 252, 262 (1920). [9-10] Not all evidence alleged to establish a condition precedent is admissible, however.

The defendant alleges the commission of prejudicial error by the court in the admission of the testimony of two witnesses, Ann Patterson and Dorothy Montgomery, the former twelve years old and the latter an infant under the age of twenty-one years, to corroborate the sworn statements of the prosecutrix, by proving, by the said witnesses, that the prosecutrix had, on previous occasions, made the same, or substantially the same, statements to them as to her relations with the accused, that she made at the trial of the case. That this is prejudicial error is made plain by the holding of this court in the cases of Brogy v. Commonwealth, 10 Gratt. (51 Va.) 722, 725; Crowson v. Swan, 164 Va. 82, 178 S.E. 898; and Meadows v. McClaugherty, 167 Va. 41, 187 S.E. 475. The admission of testimony to prove declarations made by a witness out of court in corroboration of testimony given by him on the trial of the case is error, except under certain circumstances, which are not present in this case.