Toplevel input, character 0:
> HB.structure Definition B2 :=  {M of isB2 M & isA2 M }.
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning:
pulling in dependencies: [missing_join_error_isTop] 
Please list them or end the declaration with '&'
[HB.implicit-structure-dependency,HB,elpi,default]
The command has indeed failed with message:
You must declare the hierarchy bottom-up or add a missing join. 
There are two ways out: 
- declare structure A2B1 before structure B2A1 if B2A1 inherits from it; 
- declare an additional structure that inherits from both A1 and A2 and from which A2B1 and/or B2A1 inherit.
