code: func (s *Stack) merge(r *Stack) *Stack {
  	// Assumes similar stacks have the same length.
  	out := &Stack{
  		Calls:  make([]Call, len(s.Calls)),
  		Elided: s.Elided,
  	}
  	for i := range s.Calls {
  		out.Calls[i] = s.Calls[i].merge(&r.Calls[i])
  	}
  	return out
  }
docs: // merge merges two similar Stack, zapping out differences.