Add a variant to a given type in a given module. As in the ReviewConfig item below, you specify the module name, the type name, and the type of the new variant.

Install.TypeVariant.makeRule "Types" "ToBackend" [ "ResetCounter" "SetCounter Int" ]

Then you will have

 type ToBackend
     = CounterIncremented
     | CounterDecremented
     | ResetCounter
     | SetCounter Int

where the last two variants are the ones added.