static Pair<WebNode> web_matchTriple_dollarVars(Symbol a, Symbol b, Symbol c, Web web) {
  L<WebNode> nodes = web_nodes(web);
  if (l(nodes) == 1) {
    // check self-relation
    WebNode n = first(nodes);
    if (nodeMatch_dollarVars(b, web_getRelationOpt(n, n)) == 0
      && nodeMatch_dollarVars(a, n) == 0
      && nodeMatch_dollarVars(c, n) == 0)
      ret pair(n);
  } else if (l(nodes) == 2) {
    WebNode n1 = first(nodes), n2 = second(nodes);
    if (nodeMatch_dollarVars(b, web_getRelationOpt(n1, n2)) == 0
      && nodeMatch_dollarVars(a, n1) == 0
      && nodeMatch_dollarVars(c, n2) == 0)
      ret pair(n1, n2);
    if (nodeMatch_dollarVars(b, web_getRelationOpt(n2, n1)) == 0
      && nodeMatch_dollarVars(a, n2) == 0
      && nodeMatch_dollarVars(c, n1) == 0)
      ret pair(n2, n1);
  }
  null;
}