docs: func (s *Shape) removeRef(ref *ShapeRef) {
  	r := s.refs
  	for i := 0; i < len(r); i++ {
  		if r[i] == ref {
  			j := i + 1
  			copy(r[i:], r[j:])
  			for k, n := len(r)-j+i, len(r); k < n; k++ {
  				r[k] = nil // free up the end of the list
  			} // for k
  			s.refs = r[:len(r)-j+i]
  			break
  		}
  	}
  }