code: protected void removeExternInput(InputId id) {
      CompilerInput input = getInput(id);
      if (input == null) {
        return;
      }
      checkState(input.isExtern(), "Not an extern input: %s", input.getName());
      inputsById.remove(id);
      externs.remove(input);
      Node root = checkNotNull(input.getAstRoot(this));
      if (root != null) {
        root.detach();
      }
    }
docs: Removes an input file from AST.
  @param id The id of the input to be removed.