* Write a function `unmap2` that is something like the inverse of `map2`. This function expects two arguments, the second being a sequence of elements of some type *t*. The first is a function `g` that expects a single argument of type *t* and returns a *pair* of results, rather than just one result. We want to collate these results, the first into one sequence, and the second into a different sequence. Then `unmap2` should return those two sequences. Thus if:
- g z1 # evaluates to [x1, y1]
- g z2 # evaluates to [x2, y2]
- g z3 # evaluates to [x3, y3]
+ g z1 # evaluates to (x1, y1)
+ g z2 # evaluates to (x2, y2)
+ g z3 # evaluates to (x3, y3)
Then `unmap2 (g, [z1, z2, z3])` should evaluate to `([x1, x2, x3], [y1, y2, y3])`.