That was easy...

but it breaks associativity or is it commutivity?

Anyway you can't add THREE or more things in one expression (yet)
because adding OberonInt objects returns a two-tuple.

I could adjust the semantics of ObInt to accept such two-tuples and do
add_with_carry() but that's probably more trouble than it's worth.

Just gotta be careful with math expressions, eh?

bigjoyints/big.py

@@ -20,21 +20,27 @@ class OberonInt:
         '''
         Return carry bit and new value.
         '''
-        assert isinstance(other, OberonInt)
+        if not isinstance(other, OberonInt):
+            other = OberonInt(other)
         n = self.value.value + other.value.value
         carry = not is_i32(n)
         if carry:
             n &= (2**31-1)
         return int(carry), OberonInt(n)
 
+    __radd__ = __add__
+
     def negate(self):
         # Instead of binary ops, just cheat:
         return OberonInt(-self.value.value)
 
     def __sub__(self, other):
-        assert isinstance(other, OberonInt)
+        if not isinstance(other, OberonInt):
+            other = OberonInt(other)
         return self + other.negate()
 
+    __rsub__ = __sub__
+
     def __repr__(self):
         #b = bin(self.value.value & (2**32-1))
         return f'OberonInt({self.value.value})'