Exclusion of evidence to explain or amend ambiguous document
95.  When the language used in a document is on its face ambiguous or defective, evidence may not be given of facts which would show its meaning or supply its defects.
Illustrations
     (a)  A agrees in writing to sell a horse to B for $500 or $600. Evidence cannot be given to show which price was to be given.
     (b)  A deed contains blanks. Evidence cannot be given of facts which would show how they were meant to be filled.