code: function _instanceCopy(sourceRef, copyRef, rc, copier) {
      let origIndex = rc.xStack.indexOf(sourceRef);
      if (origIndex === -1) {
          rc.push(sourceRef, copyRef);
          forEach(sourceRef, function(value, key) {
              copier(copyRef, value, key);
          });
          rc.pop();
          return copyRef;
      }
      else
          return rc.yStack[origIndex];
  }
docs: The shared instance copier based on provided callback.
  
  @param sourceRef A reference to the source.
  @param copyRef A reference to the copy.
  @param rc The current recurse and reference counter.
  @param copier The callback used to make a copy of the property list from source to copy reference.
  @returns {any} The reference to the copy.