{"ast_node_type":"variable","tokens":[{"end":51,"kind":"keyword","start":0,"text":"let"}]}
        let new_expr = Expression { lhs, op, rhs };
    fn make_expression(&mut self, lhs: CovTerm, op: Op, rhs: CovTerm) -> CovTerm {
        *self.expressions_memo.entry(new_expr.clone()).or_insert_with(|| {