#!/usr/bin/perl -w
use strict;
use warnings;

my @stack = ();

while (<DATA>) {
        chomp;

        print "TOS='" . (@stack ? $stack[$#stack] : '-nil-') . "', "
        .       "stmt = '" . $_ . "'\n";

        my ($operator, $operand) = split;
        if ($operator eq 'push') {
                push @stack, $operand;
        }
        elsif ($operator eq 'print') {
                print "out: '" . (pop @stack) . "'\n";
        }
        elsif ($operator eq 'add') {
                my $lhs = pop @stack;
                my $rhs = pop @stack;
                push @stack, '(' . $lhs . ' + ' . $rhs . ')';
        }
        elsif ($operator eq 'mul') {
                my $lhs = pop @stack;
                my $rhs = pop @stack;
                push @stack, '(' . $lhs . '*' . $rhs . ') ';
        }
}
__DATA__
push 3
push 5
add
push 7
push 9
add
mul
print