function XReduceBy(valueFn, valueAcc, keyFn, xf) {
    this.valueFn = valueFn;
    this.valueAcc = valueAcc;
    this.keyFn = keyFn;
    this.xf = xf;
    this.inputs = {};
  }
Returns the number of items in a given `list` matching the predicate `f`

@func
@memberOf R
@since v0.28.0
@category List
@sig (a -> Boolean) -> [a] -> Number
@param {Function} predicate to match items against
@return {Array} list of items to count in
@example

     const even = x => x % 2 == 0;

     R.count(even, [1, 2, 3, 4, 5]); // => 2
     R.map(R.count(even), [[1, 1, 1], [2, 3, 4, 5], [6]]); // => [0, 2, 1]