MATH.4:6 - Bias-Annotation

A familiar induction proof can tempt the writer to leave the output operation implicit. Recover the witness produced in each case and the information the next case uses.

A second temptation is to keep the initial statement fixed even after the recursive step exposes missing information. The tree case needs both root colors; strengthening that specification repairs the construction.

Simple recursion can also look like a recommended implementation. The division case states its cost so that a mathematically useful construction can be replaced for a larger computational use.