code: func Compile(s string) (expr Expression, err error) {
  	if s == "0" {
  		return constValue{value: 0}, nil
  	}
  	if !strings.Contains(s, "?") {
  		s += "?1:0"
  	}
  	return compileExpression(s)
  }
docs: // Compile a string containing a plural form expression to a Expression object.