public override HomeworkExpression[] ParseInput(string rawInput) =>
rawInput.Split('\n').Select(l => HomeworkExpression.Parse(Invert(l))).ToArray();
private static HomeworkExpression ParseOperation(string text, int index, HomeworkExpression left) =>
text.ElementAtOrDefault(index + 2) switch
{
'+' => new AdditionExpression(left, Parse(text[(index + 4)..])),
