# RetractableTo S¶

A is retractable to B means that some elements of A can be converted into elements of B and any element of B can be converted into an element of A.
coerce(a) transforms a into an element of %.
retract(a) transforms a into an element of S if possible. Error: if a cannot be made into an element of S.
retractIfCan(a) transforms a into an element of S if possible. Returns “failed” if a cannot be made into an element of S.