function _path(pathAr, obj) {
    var val = obj;
    for (var i = 0; i < pathAr.length; i += 1) {
      if (val == null) {
        return undefined;
      }
      var p = pathAr[i];
      if (_isInteger(p)) {
        val = _nth(p, val);
      } else {
        val = val[p];
      }
    }
    return val;
  }
Returns a lens whose focus is the specified index.

@func
@memberOf R
@since v0.14.0
@category Object
@typedefn Lens s a = Functor f => (a -> f a) -> s -> f s
@sig Number -> Lens s a
@param {Number} n
@return {Lens}
@see R.view, R.set, R.over, R.nth
@example

     const headLens = R.lensIndex(0);

     R.view(headLens, ['a', 'b', 'c']);            //=> 'a'
     R.set(headLens, 'x', ['a', 'b', 'c']);        //=> ['x', 'b', 'c']
     R.over(headLens, R.toUpper, ['a', 'b', 'c']); //=> ['A', 'b', 'c']