isdef — checks variable existence
isdef(name [,where])
isdef(name) returns %T if the
variable namedname exists and %F
otherwise.
Caveats: a function which uses isdef may return a
result which depends on the environment!
isdef(name,'local') returns %T
if the variable namedname exists in the local
environment of the current function and %F
otherwise.
isdef(name,'nolocal') returns
%T if the variable named name exists
in the full calling environment (including the global level) of the
current function and %F otherwise.