All classes come from some super class, you need to lookup ancestors also lookup the RUBY docs, Integer inherits methods from its superclasses.

class numeric has step method

I am new to Ruby - I'd expect the min parameter to be an integer, but the integer class doesn't have a step method. What gives?

https://ruby-doc.org/core-2.5.0/Integer.html

Afaik, len isn't taking Unicode into account.

Can someone please explain this? (I'm mostly curious about the last line.)

Cool! Thanks for the link @christianhammer!

For obvious reasons (you didn't do anything but state the opposite), I disagree.

But reading my own solution again: yes - it could be more readable.

This is much easier to read than your solution.

Thanks. It looked wrong to begin with, but it does make sense now.

Can you prove that this algorithm always returns?

Now using two block in stead of one has just gotten me really confused. I'd prefer switching back to just one or using two different chars that could add up to look like a cell in stead, like "[]".

I am probably reading this wrong, but it looks as if EmptyList is a subclass of List?

I thought about about writing a solution using regexes like this, but decided against it because it is very hard to read. But yes - its really compact.

Another thing: the string '1e1, 1e1' should be included for invalid coordinates in the test set. At least one of the completed solutions will wrongly accept this as a valid coordinate.

I found out what the problem is: my code returns a false-ish value (null) and the test requires false. I'd expect false-ish values to be acceptable as well (and the requirements doesn't state anything on this matter).