code: Choice<State<PlaceholderParamIdent>> tryBindArguments(
        final ExpressionTree node, final State<?> state) {
      return Choice.from(arguments().entrySet())
          .thenChoose(
              (final Map.Entry<UVariableDecl, UExpression> entry) ->
                  unifyParam(entry.getKey(), entry.getValue(), node, state.fork()));
    }
docs: Returns all the ways this tree might be unified with the arguments to this placeholder
  invocation. That is, if the placeholder invocation looks like {@code placeholder(arg1, arg2,
  ...)}, then the {@code Choice} will contain any ways this tree can be unified with {@code
  arg1}, {@code arg2}, or the other arguments.