docs: function an (type, msg) {
      if (msg) flag(this, 'message', msg);
      type = type.toLowerCase();
      var obj = flag(this, 'object')
        , article = ~[ 'a', 'e', 'i', 'o', 'u' ].indexOf(type.charAt(0)) ? 'an ' : 'a ';
  
      this.assert(
          type === _.type(obj)
        , 'expected #{this} to be ' + article + type
        , 'expected #{this} not to be ' + article + type
      );
    }