sub evalTree {
    my $intree = shift;
    my $param = shift;

    local $_ = $intree->{contents}; # A shortcut for the current node

    return 0 unless defined;

    if (/^sub /) {
      return eval->(
                  evalTree($intree->{left},$param),
                  evalTree($intree->{right},$param)
                 );
    
    } elsif (/^[x]$/) {
      return $param->{$_};
    } else {
      return $_;
    }
  }