Evidence of written agreements (aka parol evidence)

Section 9. — When the terms of an agreement have been reduced to writing, it is considered as containing all the terms agreed upon and there can be, between the parties and their successors in interest, no evidence of such terms other than the contents of the written agreement.

However, a party may present evidence to modify, explain or add to the terms of written agreement if he puts in issue in his pleading:

[a] An intrinsic ambiguity, mistake or imperfection in the written agreement;
[b] The failure of the written agreement to express the true intent and agreement of the parties thereto;
[c] The validity of the written agreement; or
[d] The existence of other terms agreed to by the parties or their successors in interest after the execution of the written agreement.

The term "agreement" includes wills. (Rule 130)