(a)any fact may be proved which would invalidate any document, or which would entitle any person to any decree or order relating thereto, such as fraud, intimidation, illegality, want of due execution, want of capacity in any contracting party, want or failure of consideration or mistake in fact or law;

(b)the existence of any separate oral agreement as to any matter on which a document is silent, and which is not inconsistent with its terms, may be proved. In considering whether or not this paragraph applies, the court shall have regard to the degree of formality of the document;

(c)the existence of any separate oral agreement, constituting a condition precedent to the attaching of any obligation under any such contract, grant or disposition of property, may be proved;

(d)the existence of any distinct subsequent oral agreement to rescind or modify any such contract, grant or disposition of property may be proved, except in cases in which that contract, grant or disposition of property is by law required to be in writing or has been registered according to the law in force for the time being as to the registration of documents;

(e)any usage or custom by which incidents not expressly mentioned in any contract are usually annexed to contracts of that description may be proved if the annexing of the incident would not be repugnant to, or inconsistent with, the express terms of the contract;

(f)any fact may be proved which shows in what manner the language of a document is related to existing facts.