error: features must be unique across all rule files
--> main.sml:2:0
     |
   2 | Foo = 1
     | ^ this feature is defined in multiple locations
~~~ such as:
--> (1) main.sml:3:0
     |
   3 | Foo = 2
     | ^
--> (2) main.sml:4:0
     |
   4 | Foo = 3
     | ^