static L<WebNode> webs_tripleSearch_dollarX(Symbol a, Symbol b, Symbol c, Collection<Web> webs) {
  new L<WebNode> l;
  bool isA = eq(a, "$X");
  for (Web web : webs) {
    Pair<WebNode> p = web_matchTriple_dollarVars(a, b, c, web);
    if (p != null)
      l.add(isA ? p.a : p.b);
  }
  ret l;
}

static L<WebNode> webs_tripleSearch_dollarX(Collection<Web> webs, T3<Symbol> t) {
  ret webs_tripleSearch_dollarX(t.a, t.b, t.c, webs);
}