diff --git a/src/sixtypical/callgraph.py b/src/sixtypical/callgraph.py
index 44990d2..b3a87db 100644
--- a/src/sixtypical/callgraph.py
+++ b/src/sixtypical/callgraph.py
@@ -33,6 +33,8 @@ def construct_callgraph(program):
             potential_calls = node['potentially-calls']
             if routine.name in potential_calls:
                 potentially_called_by.append(name)
+        if getattr(routine, 'explicitly_marked_as_called', None) or routine.name == 'main':
+            potentially_called_by.append('*marked*')
         graph[routine.name]['potentially-called-by'] = potentially_called_by
 
     return graph
diff --git a/tests/SixtyPical Callgraph.md b/tests/SixtyPical Callgraph.md
index 5caf5f8..aeee12b 100644
--- a/tests/SixtyPical Callgraph.md	
+++ b/tests/SixtyPical Callgraph.md	
@@ -10,14 +10,112 @@ called.
 
     -> Tests for functionality "Dump callgraph info for SixtyPical program"
 
-The `main` routine is always called.
+The `main` routine is always called.  The thing that it will
+be called by is the system, but the callgraph analyzer will
+simply consider it to be "marked as called".
 
     | define main routine
     | {
     | }
     = {
     =     "main": {
+    =         "potentially-called-by": [
+    =             "*marked*"
+    =         ],
+    =         "potentially-calls": []
+    =     }
+    = }
+
+If a routine is called by another routine, this fact will be noted.
+
+    | define main routine
+    | {
+    |   call other
+    | }
+    | 
+    | define other routine
+    | {
+    | }
+    = {
+    =     "main": {
+    =         "potentially-called-by": [
+    =             "*marked*"
+    =         ],
+    =         "potentially-calls": [
+    =             "other"
+    =         ]
+    =     },
+    =     "other": {
+    =         "potentially-called-by": [
+    =             "main"
+    =         ],
+    =         "potentially-calls": []
+    =     }
+    = }
+
+If a routine is not called by another routine, and it is not `main`
+and it is not explicitly marked as preserved, this absence will be
+noted, and a compiler or linker will be permitted to omit it from
+the final executable.
+
+    | define main routine
+    | {
+    | }
+    | 
+    | define other routine
+    | {
+    | }
+    = {
+    =     "main": {
+    =         "potentially-called-by": [
+    =             "*marked*"
+    =         ],
+    =         "potentially-calls": []
+    =     },
+    =     "other": {
     =         "potentially-called-by": [],
     =         "potentially-calls": []
     =     }
     = }
+
+If two routines potentially call each other, this will be noted,
+even if nothing else potentially calls either of those routines.
+This may change in the future.
+
+    | define main routine
+    | {
+    | }
+    | 
+    | define other1 routine
+    | {
+    |   call other2
+    | }
+    | 
+    | define other2 routine
+    | {
+    |   call other1
+    | }
+    = {
+    =     "main": {
+    =         "potentially-called-by": [
+    =             "*marked*"
+    =         ],
+    =         "potentially-calls": []
+    =     },
+    =     "other1": {
+    =         "potentially-called-by": [
+    =             "other2"
+    =         ],
+    =         "potentially-calls": [
+    =             "other2"
+    =         ]
+    =     },
+    =     "other2": {
+    =         "potentially-called-by": [
+    =             "other1"
+    =         ],
+    =         "potentially-calls": [
+    =             "other1"
+    =         ]
+    =     }
+    = }