diff --git a/src/protocol/ast.rs b/src/protocol/ast.rs
index 7657a9390ebffa00927a9cdc1bc6ae71225e0df2..b92a35cc3ebf01a92e04de55bbc52babd6d429a8 100644
--- a/src/protocol/ast.rs
+++ b/src/protocol/ast.rs
@@ -752,7 +752,7 @@ pub struct Variable {
     pub parser_type: ParserType,
     pub identifier: Identifier,
     // Validator/linker
-    pub relative_pos_in_block: u32,
+    pub relative_pos_in_block: i32,
     pub unique_id_in_scope: i32, // Temporary fix until proper bytecode/asm is generated
 }
 
@@ -1170,7 +1170,7 @@ pub struct BlockStatement {
     pub scope_node: ScopeNode,
     pub first_unique_id_in_scope: i32, // Temporary fix until proper bytecode/asm is generated
     pub next_unique_id_in_scope: i32, // Temporary fix until proper bytecode/asm is generated
-    pub relative_pos_in_parent: u32,
+    pub relative_pos_in_parent: i32,
     pub locals: Vec<VariableId>,
     pub labels: Vec<LabeledStatementId>,
     pub next: StatementId,
@@ -1224,6 +1224,7 @@ pub struct MemoryStatement {
     // Phase 1: parser
     pub span: InputSpan,
     pub variable: VariableId,
+    pub initial_expr: AssignmentExpressionId,
     // Phase 2: linker
     pub next: StatementId,
 }
@@ -1241,7 +1242,7 @@ pub struct ChannelStatement {
     pub from: VariableId, // output
     pub to: VariableId,   // input
     // Phase 2: linker
-    pub relative_pos_in_block: u32,
+    pub relative_pos_in_block: i32,
     pub next: StatementId,
 }
 
@@ -1252,7 +1253,7 @@ pub struct LabeledStatement {
     pub label: Identifier,
     pub body: StatementId,
     // Phase 2: linker
-    pub relative_pos_in_block: u32,
+    pub relative_pos_in_block: i32,
     pub in_sync: SynchronousStatementId, // may be invalid
 }
 
@@ -1357,8 +1358,9 @@ pub struct SelectStatement {
 
 #[derive(Debug, Clone)]
 pub struct SelectCase {
-    pub guard_var: MemoryStatementId, // invalid ID if there is no declaration of a variable
-    pub guard_expr: ExpressionStatementId, // if `guard_var.is_some()`, then always assignment expression
+    // The guard statement of a `select` is either a MemoryStatement or an
+    // ExpressionStatement. Nothing else is allowed by the initial parsing
+    pub guard: StatementId,
     pub block: BlockStatementId,
 }
 
@@ -1410,6 +1412,7 @@ pub struct ExpressionStatement {
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
 pub enum ExpressionParent {
     None, // only set during initial parsing
+    Memory(MemoryStatementId),
     If(IfStatementId),
     While(WhileStatementId),
     Return(ReturnStatementId),