function _stepCat(obj) {
    if (_isTransformer(obj)) {
      return obj;
    }
    if (_isArrayLike(obj)) {
      return _stepCatArray;
    }
    if (typeof obj === 'string') {
      return _stepCatString;
    }
    if (_typeof(obj) === 'object') {
      return _stepCatObject;
    }
    throw new Error('Cannot create transformer for ' + obj);
  }
Creates an object containing a single key:value pair.

@func
@memberOf R
@since v0.18.0
@category Object
@sig String -> a -> {String:a}
@param {String} key
@param {*} val
@return {Object}
@see R.pair
@example

     const matchPhrases = R.compose(
       R.objOf('must'),
       R.map(R.objOf('match_phrase'))
     );
     matchPhrases(['foo', 'bar', 'baz']); //=> {must: [{match_phrase: 'foo'}, {match_phrase: 'bar'}, {match_phrase: 'baz'}]}