static L<WebNode> thoughtSpace_search_dollarX(WebThoughtSpace ts, S a, S b, S c) {
  S query = isDollarVar(a) ? c : a;
  ret webs_search_dollarXY(webFromTriples(a, b, c),
    nodesToWebs(shortestList(
      ts.indexedNodes(a), ts.indexedNodes(b), ts.indexedNodes(c)));
}