function XFindLast(f, xf) {
    this.xf = xf;
    this.f = f;
  }
Returns the index of the first element of the list which matches the
predicate, or `-1` if no element matches.

Acts as a transducer if a transformer is given in list position.

@func
@memberOf R
@since v0.1.1
@category List
@sig (a -> Boolean) -> [a] -> Number
@param {Function} fn The predicate function used to determine if the element is the
desired one.
@param {Array} list The array to consider.
@return {Number} The index of the element found, or `-1`.
@see R.transduce, R.indexOf
@example

     const xs = [{a: 1}, {a: 2}, {a: 3}];
     R.findIndex(R.propEq(2, 'a'))(xs); //=> 1
     R.findIndex(R.propEq(4, 'a'))(xs); //=> -1