static Map<S, Number> ai_gc_distanceFunction_backwardValues = litcimap(
  "is", 1,
  [[is "a"]], 0.1,
  [[unquotes to]], 0.1,
  "probably is", 1,
);

// returns NaN if not a relevant connection
static double ai_gc_distanceFunction(S rel, bool forward) {
  if (forward) ret 1; // all forward connections matter
  
  if (!forward) {
    rel = dropSuffix(":", rel);
    Number n = ai_gc_distanceFunction_backwardValues.get(rel);
    if (n != null)
      ret toDouble(n);
  }
    
  ret nan();
}