Use more explicite garbage placholder value.

stepper.py

@@ -1,6 +1,8 @@
 import ll
 import parser
 
+from enum import Enum
+
 
 def TODO(msg):
     print('TODO: not implemented yet at {}'
@@ -17,6 +19,7 @@ def warn(msg):
           .format(msg))
 
 
+class Garbage(Enum):
     GARBAGE = '<<Unitialized memory>>'
 
 
@@ -55,7 +58,8 @@ def step(insns, terminator, blocks, stack_frames, ssa_env, global_env, memory,
 
         ptr = len(memory)
         for i in range(max(size, 1)):
-            memory.append(GARBAGE)
+            memory.append(Garbage.GARBAGE)
+
         res = ptr
     elif isinstance(next_insn, ll.Load):
         TODO('Load')