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.
220.127.116.11.4.1 Examples of for-as-equals-then subclause