function isSubsetOf(subset, superset, cmp) {
    return subset.every(function(elem) {
      if (!cmp) return superset.indexOf(elem) !== -1;

      return superset.some(function(elem2) {
        return cmp(elem, elem2);
      });
    })
  }
### .closeTo(expected, delta)

Asserts that the target is equal `expected`, to within a +/- `delta` range.

    expect(1.5).to.be.closeTo(1, 0.5);

@name closeTo
@param {Number} expected
@param {Number} delta
@param {String} message _optional_
@api public