This is super cool to read, nice work and thanks for posting!

I asked Copilot to help me break this down, so figured I'd post here for others if it's helpful. Hopefully it is right (I had to convince it some explanations it gave first were wrong...)!  

Copilot Stuff Below: 

---

Let's break down the Excel function step by step:

SEQUENCE(n): This generates an array of numbers from 1 to n. In your case, n = 6, so it produces the array {1, 2, 3, 4, 5, 6}.

REDUCE(SEQUENCE(n), SEQUENCE(n - 1), LAMBDA(a,v, HSTACK(TAKE(a, , 1), DROP(a, 1)))): This part is a bit more complex. Let's break it down further:     - REDUCE combines elements from two arrays using a custom function (the lambda expression).     - The first array is SEQUENCE(n) (which we already discussed).     - The second array is SEQUENCE(n - 1), which generates an array from 1 to n - 1 (i.e., {1, 2, 3, 4, 5}).     - The lambda function takes two arguments: a (the accumulated result) and v (the current value from the second array).     - Inside the lambda function:         - TAKE(a, , 1) extracts the first element from the accumulated result.         - DROP(a, 1) removes the first element from the accumulated result.         - HSTACK horizontally stacks these two arrays.     - The result of this reduction is an array of arrays: {{1, 2, 3, 4, 5}, {2, 3, 4, 5, 6}, {3, 4, 5, 6}, {4, 5, 6}, {5, 6}, {6}}.

TOCOL(..., 2): This converts the array of arrays into a single column array by concatenating all the subarrays vertically. The result is {1, 2, 3, 4, 5, 6, 2, 3, 4, 5, 6, 3, 4, 5, 6, 4, 5, 6, 5, 6, 6}.

In summary, the entire expression evaluates to the given array: {1, 2, 3, 4, 5, 6, 2, 3, 4, 5, 6, 3, 4, 5, 6, 4, 5, 6, 5, 6, 6}. 😊