Cast converts an element of type X to an interface of type Y.

Range calls some function f(k, v) => err over any [Rangeable] of (K, V)s.

WithCloser ..

Rangeable defines any type of value x where it is possible to range over using "for k, v := range x" or "v := range x" (in the case of a channel, only "v := range x" is permitted).