Internal marker wrapping a { ... } block's already-flattened entries, distinguishing "this statement's right-hand side was a block" from "was a scalar value" (Cooper.Actions' own handle_rule/3 clause for :kv_statement branches on it). A struct rather than a tagged tuple deliberately -- CASC tuples (§6.11) are real Elixir tuples built from user data, and a plain {:block_ops, list} tag could theoretically collide with one; a struct never can.