docs: private ImmutableList<UTypeVar> freeTypeVars(Unifier unifier) {
      ImmutableList.Builder<UTypeVar> builder = ImmutableList.builder();
      for (UTypeVar var : typeVariables(unifier.getContext())) {
        if (unifier.getBinding(var.key()) == null) {
          builder.add(var);
        }
      }
      return builder.build();
    }