What is the collateral fact rule in law and what is the effect of contradictory evidence?

The Collateral Fact Rule is summarized in Watts Manual of Criminal Evidence as follows: The collateral facts or collateral issues rule prohibits the introduction of evidence for the sole purpose of contradicting a witness' testimony concerning a collateral fact. The rule seeks to avoid confusion and proliferation of issues, wasting of time and introduction of evidence of negligble assistance to the trier of fact in determining the real issues of the case. It endeavours to ensure that the sideshow does not take over the circus. In general, matters that relate wholly and exclusively to the credibility of a non-accused witness are collateral, hence beyond the reach of contradictory evidence. A collateral fact is one that is not connected with the issue in the case. It is one that the party would not be entitled to prove as part of its case, because it lacks relevance or connection to it. A collateral fact, in other words, is one that is neither i. material; nor ii. relevant to a material fact. If the answer of a witness that a party seeks to contradict, is a matter that the opponent could prove in evidence as part of its case, independent of the contradiction, the matter is not collateral. Contradictory evidence may be elicited. If the answer of a witness that a party seeks to contradict, is a matter that the opponent would not be entitled to prove in evidence, independent of the contradiction, the matter is collateral. Contradictory proof is not permitted. The several exceptions to the collateral facts rule include and permit: i. proof of bias, interest or corruption denied by the witness; ii. proof of a prior inconsistent statement not admitted by the witness; iii. proof of a prior conviction not admitted by the witness; iv. disproof of a denial of a reputation for lying or untruthfulness; and v. disproof of a denial of a physical or mental defect relating to the capacity of likelihood of the witness telling the truth.