default FunctionalCommandBuilder<T> assertArgument(int index, Predicate<String> test) {
        return assertArgument(index, test, DEFAULT_INVALID_ARGUMENT_MESSAGE);
    }
Tests a given argument with the provided predicate.

The default failure message is sent if the argument does not pass the predicate. If the argument is not
present at the given index, <code>null</code> is passed to the predicate.

@param index the index of the argument to test
@param test  the test predicate
@return the builder instance
helper/src/main/java/me/lucko/helper/command/functional/FunctionalCommandBuilder.java