let sourceL = getLineage(sourceNode);
if (isIterator(targetNode)) {
  // the inputs of iterator nodes only allow non-iterator connections
  return sourceL === undefined ? CONNECTABLE : INVALID;
}
if (isRegularNode(targetNode)) {
  if (sourceL === undefined) return CONNECTABLE;
  let targetL = getLineage(targetNode, excluded=targetInput)
  return targetL === undefined || targetL === sourceL ? CONNECTABLE : INVALID;
}
assert(isCollector(targetNode));
if (isIteratorInput(targetInput)) {
  if (sourceL === undefined) return INVALID;
  let targetL = getLineage(targetNode.iteratorInputs, excluded=targetInput)
  return targetL === undefined || targetL === sourceL ? CONNECTABLE : INVALID;
} else {
  // the non-iterator inputs of collector nodes only allow non-iterator connections
  return sourceL === undefined ? CONNECTABLE : INVALID;
}