###
### memoize
###

## Chapter 3 section 5

sub memoize {
  my ($func) = @_;
  my %cache;
  my $stub = sub {
    my $key = join ',', @_;
    $cache{$key} = $func->(@_) unless exists $cache{$key};
    return $cache{$key};
  };
  return $stub;
}