diff --git a/projects/shoggoth/src/python/ichor/assembler.py b/projects/shoggoth/src/python/ichor/assembler.py
index fb36db9..2aa2c1c 100644
--- a/projects/shoggoth/src/python/ichor/assembler.py
+++ b/projects/shoggoth/src/python/ichor/assembler.py
@@ -1,5 +1,6 @@
 #!/usr/bin/env python3
 
+import logging
 from random import choices
 from string import ascii_lowercase, digits
 from typing import List, Optional, Sequence, Union
@@ -7,6 +8,9 @@ from typing import List, Optional, Sequence, Union
 from ichor import isa
 
 
+log = logging.getLogger(__name__)
+
+
 def gensym(prefix = None) -> isa.Label:
     frag = "".join(choices(ascii_lowercase + digits, k=8))
     return isa.Label(f"{prefix or 'gensym'}_{frag}")
@@ -26,22 +30,14 @@ class FuncBuilder(object):
         if not self._straightline:
             return
 
-        start_stack = self._stack
         match op:
-            case isa.Label(_):
-                # Labels abort
-                self._straightline = False
-
-            case isa.GOTO(_) | isa.ATEST(_):
-                # Control ops abort
+            case isa.Label(_) | isa.GOTO(_) | isa.ATEST(_):
+                # Labels and control ops abort
                 self._straightline = False
 
             case isa.CLOSUREC(n) | isa.CLOSUREF(n):
                 self._stack -= n
 
-            case isa.TYPEREF():
-                pass
-
             case isa.ARMREF():
                 self._stack -= 1
 
@@ -51,14 +47,15 @@ class FuncBuilder(object):
             case isa.DUP(n):
                 self._stack += n
 
-            case isa.ROT(_):
-                pass
-
-            case _:
+            case isa.IDENTIFIERC(_) | isa.SLOT(_) | isa.CALLF(_) | isa.CALLC(_) | isa.ARM(_):
                 self._stack -= getattr(op, "nargs", 0)
                 self._stack += 1
 
-        print(op, "pre", start_stack, "post", self._stack)
+            case isa.ROT(_) | isa.FUNREF(_) | isa.TYPEREF() | isa.ALOAD():
+                pass
+
+            case _:
+                log.debug("No stack manipulation known for %s, assuming no change", op)
 
     def write(self, op: Union[isa.Opcode, isa.Label, Sequence[isa.Opcode]]):
 
diff --git a/projects/shoggoth/src/python/ichor/isa.py b/projects/shoggoth/src/python/ichor/isa.py
index 3667ffc..ea06037 100644
--- a/projects/shoggoth/src/python/ichor/isa.py
+++ b/projects/shoggoth/src/python/ichor/isa.py
@@ -2,6 +2,7 @@
 
 from abc import ABC
 from dataclasses import dataclass
+from typing import Union
 
 
 @dataclass
@@ -26,7 +27,7 @@ class GOTO(Opcode):
 
     """
 
-    target: int
+    target: Union[int, Label]
 
 ################################################################################
 # Stack manipulation
@@ -77,7 +78,7 @@ class SLOT(Opcode):
 
     """
 
-    target: int
+    target: Union[int, Label]
 
 ################################################################################
 # Functional abstraction
@@ -227,7 +228,7 @@ class ATEST(Opcode):
 
     """
 
-    target: int
+    target: Union[int, Label]
 
 
 @dataclass
diff --git a/projects/shoggoth/test/python/ichor/test_assembler.py b/projects/shoggoth/test/python/ichor/test_assembler.py
index a6feb69..8ed747f 100644
--- a/projects/shoggoth/test/python/ichor/test_assembler.py
+++ b/projects/shoggoth/test/python/ichor/test_assembler.py
@@ -49,7 +49,7 @@ def test_self_label(builder: FuncBuilder):
 
 
 def test_local_label(builder: FuncBuilder):
-    s0 = builder.mark_slot()
+    s0 = builder.mark_slot(target=0)
     builder.write(isa.SLOT(s0))
 
     s999 = builder.mark_slot(target=999)
diff --git a/projects/shoggoth/test/python/ichor/test_bootstrap.py b/projects/shoggoth/test/python/ichor/test_bootstrap.py
index c3f793d..596fd38 100644
--- a/projects/shoggoth/test/python/ichor/test_bootstrap.py
+++ b/projects/shoggoth/test/python/ichor/test_bootstrap.py
@@ -4,13 +4,13 @@ from .fixtures import *  # noqa
 
 from ichor import isa
 from ichor.bootstrap import (
+    AND2,
+    AND3,
     FALSE,
     NOT1,
     OR2,
     OR3,
     TRUE,
-    AND2,
-    AND3
 )
 import pytest