code: function _setCopy(sourceRef, copyRef, rc) {
      return _instanceCopy(sourceRef, copyRef, rc, (set, val) => {
          set.add(_clone(val, rc));
      });
  }
docs: 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.