diff --git a/build/ab.py b/build/ab.py
index 5ad3ca8f..92fe324d 100644
--- a/build/ab.py
+++ b/build/ab.py
@@ -32,7 +32,8 @@
 
 class PathFinderImpl(PathFinder):
     def find_spec(self, fullname, path, target=None):
-        if not path:
+        # The second test here is needed for Python 3.9.
+        if not path or not path[0]:
             path = ["."]
         if len(path) != 1:
             return None
@@ -114,7 +115,7 @@ def wrapper(*, name=None, replaces=None, **kwargs):
         t.callback = func
         t.traits.add(func.__name__)
         if "args" in kwargs:
-            t.args |= kwargs["args"]
+            t.args.update(kwargs["args"])
             del kwargs["args"]
         if "traits" in kwargs:
             t.traits |= kwargs["traits"]
@@ -354,8 +355,15 @@ def convert(value, target):
         return output
 
 
+def _removesuffix(self, suffix):
+    # suffix='' should not call self[:-0].
+    if suffix and self.endswith(suffix):
+        return self[:-len(suffix)]
+    else:
+        return self[:]
+
 def loadbuildfile(filename):
-    filename = filename.replace("/", ".").removesuffix(".py")
+    filename = _removesuffix(filename.replace("/", "."), ".py")
     builtins.__import__(filename)
 
 
diff --git a/build/c.py b/build/c.py
index 0045e634..f84fea5d 100644
--- a/build/c.py
+++ b/build/c.py
@@ -81,6 +81,12 @@ def cxxfile(
     cfileimpl(self, name, srcs, deps, suffix, commands, label, cflags)
 
 
+def _removeprefix(self, prefix):
+    if self.startswith(prefix):
+        return self[len(prefix):]
+    else:
+        return self[:]
+
 def findsources(name, srcs, deps, cflags, filerule, cwd):
     for f in filenamesof(srcs):
         if f.endswith(".h") or f.endswith(".hh"):
@@ -90,7 +96,7 @@ def findsources(name, srcs, deps, cflags, filerule, cwd):
     for s in flatten(srcs):
         objs += [
             filerule(
-                name=join(name, f.removeprefix("$(OBJ)/")),
+                name=join(name, _removeprefix(f,"$(OBJ)/")),
                 srcs=[f],
                 deps=deps,
                 cflags=sorted(set(cflags)),
@@ -102,6 +108,8 @@ def findsources(name, srcs, deps, cflags, filerule, cwd):
             or f.endswith(".cpp")
             or f.endswith(".S")
             or f.endswith(".s")
+            or f.endswith(".m")
+            or f.endswith(".mm")
         ]
         if any(f.endswith(".o") for f in filenamesof([s])):
             objs += [s]
diff --git a/src/arch/snes/build.py b/src/arch/snes/build.py
index 9d6b5c37..956a49b7 100644
--- a/src/arch/snes/build.py
+++ b/src/arch/snes/build.py
@@ -29,7 +29,7 @@
     items={
         "0:ccp.sys@sr": "src+ccp",
         "0:bdos.sys@sr": "src/bdos",
-        "0:cls.com": "apps+cls",
+        "0:cls.com": "apps+cls"
     }
     | MINIMAL_APPS
     | MINIMAL_APPS_SRCS