> MAIL added flag -sequence-unit {false|true}
>
> If true, then in the sequence expression (e1; e2), it is a type
> error if e1 is not of type unit.
Any objection to using the same flag to signal an error when the
body of a while expression is not of type unit? (Not that I've ever seen
a while expression in an SML program, but you never know.)