code: func (cs callSet) Failures() []*Call {
  	failures := make([]*Call, 0, len(cs.expected))
  	for _, calls := range cs.expected {
  		for _, call := range calls {
  			if !call.satisfied() {
  				failures = append(failures, call)
  			}
  		}
  	}
  	return failures
  }
docs: // Failures returns the calls that are not satisfied.