code: func (t *translator) C(key interface{}, num float64, digits uint64, param string) (string, error) {
  
  	tarr, ok := t.cardinalTanslations[key]
  	if !ok {
  		return unknownTranslation, ErrUnknowTranslation
  	}
  
  	rule := t.CardinalPluralRule(num, digits)
  
  	trans := tarr[rule]
  
  	b := make([]byte, 0, 64)
  	b = append(b, trans.text[:trans.indexes[0]]...)
  	b = append(b, param...)
  	b = append(b, trans.text[trans.indexes[1]:]...)
  
  	return string(b), nil
  }
docs: // C creates the cardinal translation for the locale given the 'key', 'num' and 'digit' arguments and param passed in