Include order with aspect orientated design

What:

Assume there is a type defined in common_file.osc, which introduces the struct Base:

# common_file.osc
struct Base

This is imported in file a.osc and b.osc and extended in both:

# a.osc
extend Base:
   new_member_a: int

# b.osc
extend Base:
   new_member_b: int

Now both, a.osc and b.osc are imported in final.osc:

# final.osc

import a.osc
import b.osc

my_base: Base

Question: Is that valid? If so, which extended version of Base should it be?