# compute assoc of free vars in the expression and print it

traversal freevars($set)
from Exp to Var
  for Derived only t,e
do
initialize:
	%$set = ();
Var.post:
	$set->{$self->{s}}++;
Lambda.post:
	delete $set->{$self->{var}};
finalize:
	my ($x);
	foreach $x (keys %$set) {
		print "free variable: $x\n";
	}
endtraversal
my ($v1,$v2,$comb,$lam1);

$v1 = new Var (s => 'v1');
$v2 = new Var (s => 'v2');
$comb = new Comb (l => $v1, r => $v2, comb_flag => 0);
$lam1 = new Lambda (var => 'v1', body => $comb);

$lam1->display(\*STDOUT);
$lam1->freevars({});