docs: private static void markEntityVars(HeadedSyntacticCategory cat, int[] uniqueVars,
        boolean[] isEntityVar) {
      if (cat.isAtomic()) {
        for (int var : cat.getUniqueVariables()) {
          int index = Ints.indexOf(uniqueVars, var);
          isEntityVar[index] = true;
        }
      } else {
        markEntityVars(cat.getArgumentType(), uniqueVars, isEntityVar);
        markEntityVars(cat.getReturnType(), uniqueVars, isEntityVar);
      }
    }