sub noop { 
   X::is_x( $_[0] );     # ok
   return $_[0];
 }
 X::is_x( noop($x) );    # not_ok

##</code><code>##

 sub noop_lval :lvalue { $_[0] }

 X:is_x( noop_lval($x) );                # ok !

 $m=20; print noop_lval(++$m) + $m++;    # 42 (still)

 $m=20; (++$m) + $m++;                   # 43