exists - checks variable existence
exists(name) returns 1 if the variable named name exists and 0 otherwise.
Caveats: a function which uses exists may return a result which depends on the environment!
exists(name,'local') returns 1 if the variable named name exists in the local environment of the current function and 0 otherwise.
deff('foo(x)',..
['disp([exists(''a12''),exists(''a12'',''local'')])'
'disp([exists(''x''),exists(''x'',''local'')])'])
foo(1)
a12=[];foo(1)