Module ast 

Operator-tree comprehension AST — spec §3.

Six constructors closed under composition: one source (clause), three combinators (cartesian, zip, union), two modifiers (filter, order). Every comprehension AST is a tree whose nodes are one of these six variants.

Closure (spec §4.1):

• C1 — every constructor returns and consumes Comprehension. There is no auxiliary value type at the AST level.
• C2 — well-formedness is decidable in one bottom-up pass (the validate module of the runtime implements the check).

Enums

Comprehension

The six-variant operator-tree comprehension.