In the for-as-equals-then subclause
the for
or as
construct
initializes the variable var by setting it to the
result of evaluating form1 on the first iteration, then setting
it to the result of evaluating form2 on the second and
subsequent iterations. If form2 is omitted, the construct
uses form1 on the second and
subsequent iterations.
The loop keywords = and then
serve as valid prepositions
in this syntax.
This construct does not provide any termination tests.