REGROUP should generate procedures to rearrange the elements in a list
according to a pattern.  Sample usage:

   > (define pairup (regroup '((1 2) (3 4) ...)))

(Yes you really type "..."!)  The argument to REGROUP is the result that you
want your procedure to generate, supposing that its argument is the list
(1 2 3 4 5 6 7 ...)

   > (pairup '(the rain in spain stays mainly on the plain))
   ((the rain) (in spain) (stays mainly) (on the))

Note that PAIRUP handles odd-length argument lists appropriately.  More
examples of patterns that REGROUP can handle:

   > (define overlap (regroup '((1 2) (2 3) ...)))
   > (overlap '(the rain in spain stays mainly on the plain))
   ((the rain) (rain in) (in spain) (spain stays) (stays mainly)
    (mainly on) (on the) (the plain))

   > (define tails (regroup '((1 2 ...) (2 3 ...) ...)))
   > (tails '(the rain in spain stays mainly on the plain))
   ((the rain in spain stays mainly on the plain)
    (rain in spain stays mainly on the plain)
    (in spain stays mainly on the plain)
    (spain stays mainly on the plain)
    (stays mainly on the plain)
    (mainly on the plain)
    (on the plain)
    (the plain)
    (plain))

   > (define swap (regroup '(2 1 4 3 ...)))
   > (swap '(the rain in spain stays mainly on the plain))
   (rain the spain in mainly stays the on)

   > (define split (regroup '((1 3 ...) (2 4 ...))))
   > (split '(the rain in spain stays mainly on the plain))
   ((the in stays on plain) (rain spain mainly the))

If you only give one example before an ellipsis, the recursion will cdr past
the number of elements required to make one instance of the exemplified
pattern.  If you give two examples, the recursion will cdr past the number of
elements equal to the difference between the first numbers of the two
examples.  (That's how OVERLAP works.)  For example,
   > (define tails (regroup '((1 ...) ...)))
is equivalent to the earlier version of TAILS.  If you use more than two
examples, the differences must agree, or else your several examples will be
taken as one big example that perhaps uses some list elements more than once.