where — get current instruction calling tree
[linenum,mac]=where()
returns linenum
and mac
such as current instruction has been
called by the linenum(1) line of function mac(1),
mac(1) has been called by the linenum(2) line of function mac(2)
and so on
mac(i) is in general the name of a function but it may also be "exec" or "execstr" if instruction lies in ans exec file or an execstr instruction