code: function(str, substr, index) {
      index = isUndefined(index) ? str.length : index;
      return str.slice(0, index) + substr + str.slice(index);
    }
docs: *
  @method insert(str, [index] = length)
  @returns String
  @short Adds `str` at [index]. Allows negative values.
  
  @example
  
  'dopamine'.insert('e', 3)       -> dopeamine
  'spelling eror'.insert('r', -3) -> spelling error
  
  @param {string} str
  @param {number} [index]
  
  *