code: func (s *callStack) popIfExists(whichFrame int) {
  	if len(s.stack) == whichFrame {
  		if s.top().isCall {
  			s.calls--
  		}
  		s.stack = s.stack[:len(s.stack)-1]
  	}
  }
docs: // It might've been popped already by tail call optimization.
  // We check if it was trimmed by comparing the current stack size to the position
  // of the frame we want to pop.