Painless CSSStyleSheet instances' rules manipulator

CSSSTyleSheet.js

// Author: hinell@github.com
// Distributed under the MIT License

// Extension for CSSStyleSheet ($('style').sheet) class that enables painless manipulation of  
// your css style sheets. 
// Be careful! This module extendes built-in prototype! 

// cssStyleSheet.rule(a?:number | string, i?: number): CSSRule | undefined
// x:number           - the "x" index of cssStyleSheet.cssRules at which the CSSRule to be deleted
// x:string           - new "x" CSSRule to add at last of the .cssRules index
// x:string, i:number - new "x" CSSRule to add into the .cssRules at "i" index
// no arguments       - remove last CSSRule from the .cssRules
CSSStyleSheet &&
CSSStyleSheet.prototype.rule || (
CSSStyleSheet.prototype.rule = function (first,second){
 var err         = function(msg){ throw msg}
    ,firstArgStr = typeof first === 'string'
    ,firstArgNum = typeof first === 'number'
    ,seconArgNum = typeof second === 'number'
    ,cssrule;
  switch (arguments.length) {
    case 0:
      cssrule = this.cssRules[this.cssRules.length - 1];
      this.cssRules.length && this.deleteRule(this.cssRules.length - 1);
      return cssrule;
      break;
    case 1:

      return firstArgStr
        ? (this.insertRule(first, this.cssRules.length ),this.cssRules[this.cssRules.length-1])
        : (firstArgNum
           ? (cssrule = this.cssRules[first]
              ,this.cssRules.length && this.deleteRule(first), cssrule)
           : err('Invalid argument: string or number is expected!')
        );
      break;
    default:
      return firstArgStr && seconArgNum
        ? (this.insertRule(first,second),this.cssRules[this.cssRules.length-1])
        : err('Invalid arguments: (string,number) is expected!');

  }
});