code: function _hash() {
      var set = Array.prototype.map.call(arguments, function(v) {
          return typeof v === 'string' ? v : Object.keys(v).join(' ');
      }).join(' ');
  
      return set.split(/\s+/)
          .reduce(function(res, keyword) {
              res[keyword] = true;
              return res;
          }, {});
  }
docs: Generates hash from strings
  
  @private
  @param {...String|KeywordsHash} keywords - Space-delimited string or previous result of _hash
  @return {KeywordsHash} - Object with keywords in keys and true in values