cast Defines a type cast in the name space converter set.
void cast(char* k, char* match, char* conv)
Cast installs a new entry in the converter set of the current name space to specify that the type k can be converted to the type match by means of the converter conv. After the call, when operating (e.g. copying) a box of type k with another of type match, both types are considered compatible types if conv is nil; if it
was not nil, the system starts conv to operate the coverted data with the destination box.
Cast overwrittes any previous entry for the same k and match.