[S4M]

I can somewhat make sense of that code, but the code of function ch-hof is a bit hard to read - and the function name is not helping. I guess he's trying to see if the king is under check, and if so by which piece(s).

Also I notice code like:

    (- (cdr kpos) 1))

How is that possible? If kpos is a list of two elements to represent the king's position, so (cdr kpos) should be a list, not compatible with subtraction.

For example:

    #;4> (- (cdr '(1 2)) 1)

    Error: (-) bad argument type: (2)

[gus_massa]

The lists are made of cons, but you can make arbitrary trees with cons.

After looking carefully, the kpos is the result of the function find. In this case it's a cons with the row and the column of the king. So the code that runs is more like

  (define kpos (cons 1 2))
  (- (cdr kpos) 1)

In this case, for clarity I prefer to use a struct instead of a cons to save the position

  (struct cell (row col))
  (define kpos (cell 1 2))
  (- (cell-col kpos) 1)

[plorkyeran]

kpos is a pair, not a list (or if you prefer, is an improper list). It's (cons 1 2), while '(1 2) is (cons 1 (cons 2 empty)).