code: @Nullable
    private static ImmutableList<Node> collectCallableLeaves(Node expr) {
      ArrayList<Node> callables = new ArrayList<>();
      boolean allLegal = collectCallableLeavesInternal(expr, callables);
      return allLegal ? ImmutableList.copyOf(callables) : null;
    }
docs: Traverses an {@code expr} to collect nodes representing potential callables that it may resolve
  to well known callables.
  
  @see {@link #collectCallableLeavesInternal}
  @return the disovered callables, or {@code null} if an unexpected possible value was found.