function _isNumber(x) {
    return Object.prototype.toString.call(x) === '[object Number]';
  }
Returns the position of the last occurrence of an item in an array, or -1 if
the item is not included in the array. [`R.equals`](#equals) is used to
determine equality.

@func
@memberOf R
@since v0.1.0
@category List
@sig a -> [a] -> Number
@param {*} target The item to find.
@param {Array} xs The array to search in.
@return {Number} the index of the target, or -1 if the target is not found.
@see R.indexOf, R.findLastIndex
@example

     R.lastIndexOf(3, [-1,3,3,0,1,2,3,4]); //=> 6
     R.lastIndexOf(10, [1,2,3,4]); //=> -1