code: function mergeField(memo, key){
    var result = {},
      match;
  
    if(_.isPlainObject(key)){
      result = key;
    }else if(_.isString(key) && (match = key.match(re.RENAME))){
      result[match[2]] = match[1];
    }else{
      result[key] = null;
    }
  
    return _.merge(memo, result);
  }
docs: Parse a fieldspec, like ["field", "to<from", {"to": "from"}]