###
### AST_to_string
###

## Chapter 2 section 2

sub AST_to_string {
  my ($tree) = @_;
  if (ref $tree) {
    my ($op, $a1, $a2) = @$tree;
    my ($s1, $s2) = (AST_to_string($a1),
                     AST_to_string($a2));
    "($s1 $op $s2)";
  } else {
    $tree;
  }
}