Representation of
This is a product type, introduced by deftagsum in support of vl-stmt.
wait (<condition>) <body>
See Section 9.7.6 (page 136). The wait statement first evaluates condition. If the result is true, body is executed. Otherwise, this flow of execution blocks until condition becomes 1, at which point it resumes and body is executed. There is no discussion of what happens when the condition is X or Z. I would guess it is treated as 0 like in if statements, but who knows.