CCBijection.Sval empty : tval is_empty : t -> boolAdd left and right correspondence to bijection such that left and right are unique in their respective sets and only correspond to each other.
val cardinal : t -> intNumber of bindings. O(n) time.
Remove the left, right binding if it exists. Return the same bijection otherwise.
Remove the binding with left key if it exists. Return the same bijection otherwise.
Remove the binding with right key if it exists. Return the same bijection otherwise.