1/3: Building A (A.idr)
2/3: Building B (B.idr)
3/3: Building C (C.idr)
Error: While processing right hand side of foo. Can't find an implementation for Monad (StateT AFoo Identity).

C:14:5--14:12
 10 | 
 11 | foo : X ()
 12 | foo =
 13 |   do
 14 |     pure ()
          ^^^^^^^

Possible cause: Undefined name A.AFoo.