diff --git a/bin/mechanicaldiff.py b/bin/mechanicaldiff.py
index 54441fc..4a8ed1f 100755
--- a/bin/mechanicaldiff.py
+++ b/bin/mechanicaldiff.py
@@ -4,6 +4,20 @@ import sys
 
 
 def should_include_change(change_lines: list[str], search: str, replace: str) -> bool:
+    removed_lines = []
+    added_lines = []
+    phase = "minus"
+    for line in change_lines:
+        if line.startswith("-"):
+            if phase != "minus":
+                raise ValueError("Non-consecutive '-' and '+' lines in change block.")
+            removed_lines.append(line[1:])
+            continue
+        if line.startswith("+"):
+            phase = "plus"
+            added_lines.append(line[1:])
+            continue
+        raise ValueError("Unexpected non-change line in change block.")
     return True