diff --git a/aider/coders/base_coder.py b/aider/coders/base_coder.py
index 3f5fa5dde..da998bfcb 100755
--- a/aider/coders/base_coder.py
+++ b/aider/coders/base_coder.py
@@ -31,6 +31,10 @@ class ExhaustedContextWindow(Exception):
     pass
 
 
+def wrap_fence(name):
+    return f"<{name}>", f"</{name}>"
+
+
 class Coder:
     abs_fnames = None
     repo = None
@@ -40,16 +44,6 @@ class Coder:
     functions = None
     total_cost = 0.0
 
-    fences = [
-        ("```", "```"),
-        ("<source>", "</source>"),
-        ("<code>", "</code>"),
-        ("<pre>", "</pre>"),
-        ("<codeblock>", "</codeblock>"),
-        ("<sourcecode>", "</sourcecode>"),
-    ]
-    fence = fences[0]
-
     @classmethod
     def create(
         self,
@@ -249,6 +243,17 @@ class Coder:
 
         self.repo = repo
 
+    # fences are obfuscated so aider can modify this file!
+    fences = [
+        ("``" + "`", "``" + "`"),
+        wrap_fence("source"),
+        wrap_fence("code"),
+        wrap_fence("pre"),
+        wrap_fence("codeblock"),
+        wrap_fence("sourcecode"),
+    ]
+    fence = fences[0]
+
     def choose_fence(self):
         all_content = ""
         for fname in self.abs_fnames:
@@ -256,14 +261,12 @@ class Coder:
 
         all_content = all_content.splitlines()
 
+        good = False
         for fence_open, fence_close in self.fences:
+            if fence_open in all_content or fence_close in all_content:
+                continue
             good = True
-            for line in all_content:
-                if line.startswith(fence_open) or line.startswith(fence_close):
-                    good = False
-                    break
-            if good:
-                break
+            break
 
         if good:
             self.fence = (fence_open, fence_close)
diff --git a/aider/diffs.py b/aider/diffs.py
index 4de55b924..730e43e1b 100644
--- a/aider/diffs.py
+++ b/aider/diffs.py
@@ -47,8 +47,8 @@ def diff_partial_update(lines_orig, lines_updated, final=False, fname=None):
     partially complete update.
     """
 
-    dump(lines_orig)
-    dump(lines_updated)
+    # dump(lines_orig)
+    # dump(lines_updated)
 
     assert_newlines(lines_orig)
     assert_newlines(lines_orig)
@@ -100,7 +100,6 @@ def diff_partial_update(lines_orig, lines_updated, final=False, fname=None):
 
     # print(diff)
 
-    dump(repr(show))
     return show