Copies data from one set to another.

@param sourceRef A reference to the set source.
@param copyRef A reference to the set copy.
@param rc The current recurse and reference counter.
@returns {Set} The reference to the set copy.
function _setCopy(sourceRef, copyRef, rc) {
    return _instanceCopy(sourceRef, copyRef, rc, (set, val) => {
        set.add(_clone(val, rc));
    });
}