The foreign function succeeds while leaving a choice point. On
backtracking over this goal the foreign function will be called again,
but the control argument now indicates it is a `Redo' call and the macro PL_foreign_context()
returns the handle passed via
This handle is a signed value two bits smaller than a pointer, i.e., 30
or 62 bits (two bits are used for status indication). Defined as return
See also PL_succeed().