sub evalRna {
    my ( $left, $right ) = @_;
    my %bonds    = ( GU => 1, UG => 1, AU => 2, UA => 2, CG => 3, GC => 3 );
    my $numBonds = $bonds{ $bases[$left] . $bases[$right] };
    my $level    = 0;
    my $new_left = $left;
    my $numRna;
    for ( my $i = $left + 1 ; $i <= $right ; $i++ ) {
        $level-- if ( $structure[$i] eq ')' );
        if ( $level == 0 ) {
            if ( $structure[$i] eq ')' ) {
                if ( $numRna = evalRna( $new_left, $i ) ) {
                    $numBonds += $numRna;
                }
            }
            $new_left = $i;
        }
        $level++ if ( $structure[$i] eq '(' );
    }
    return $numBonds;
}