Interesting! I was not aware of the range syntax with 3 dots, where 1...n exludes the end of the range.

That's bitwise xor assignment equivalent to x = x ^ 1

what is the assignment operator ^= ?

We need a simple test that a non-array element in the first is not the same as an array in the second.

Test.expect( ![1,1].same_structure_as([2,[2,2]]), "[1,1] not same as [2,[2,2]]")

This solution is incomplete... it's not catching the case where the first array has a non-array element and the second array does.

For example, this test will not pass:

Test.expect( ![1,1].same_structure_as([2,[2,2]]), "[1,1] not same as [2,[2,2]]")

The test cases are too weak.

I've never seen an array being expanded (by push) while iterating through it with each.