So, I'm not exactly sure what your question is; it almost sounds like you're asking me to explain unification in general, or maybe pattern-matching in general. But there are much better sources than me for understanding those things, so instead I'll just try giving a high-level overview of what happens with my code when we run the goal

`(zipo '((1 3 5) (2 4 6)) q)`

, where

`q`

is a fresh logic variable.

In the first clause of the

`matche`

expression, we try to unify the list

`(() ())`

with

`((1 3 5) (2 4 6))`

, which fails, so we fall through to the next clause. There we try to unify

`((X . Xs) (Y . Ys))`

with

`((1 3 5) (2 4 6))`

, which works fine since

`X`

,

`Xs`

,

`Y`

, and

`Ys`

are all fresh logic variables. In particular, we unify

`1`

with

`X`

,

`(3 5)`

with

`Xs`

,

`2`

with

`Y`

, and

`(4 6)`

with

`Ys`

.

We also unify

`q`

with

`((X Y) . XYs)`

, which, since we've already got associations for

`X`

and

`Y`

, means that we're actually unifying

`q`

with

`((1 2) . XYs)`

where the variable

`XYs`

is still fresh. Finally, we make a recursive call to

`zipo`

with the arguments

`((3 5) (4 6))`

and

`XYs`

. Lather, rinse, repeat. Eventually we reach the base case and

`zipo`

's second argument gets unified with

`()`

.

Again, I'm not sure if this was really the explanation you were looking for. If you're familiar with, say, Prolog, none of this should be very surprising. (For comparison, Chris did it in Prolog

here.)