mod40.hs:3:1: error:
    • Superclass cycle for ‘C1’
        one of whose superclasses is ‘C2’
        one of whose superclasses is ‘C1’
      Use UndecidableSuperClasses to accept this
    • In the class declaration for ‘C1’

mod40.hs:4:1: error:
    • Superclass cycle for ‘C2’
        one of whose superclasses is ‘C1’
        one of whose superclasses is ‘C2’
      Use UndecidableSuperClasses to accept this
    • In the class declaration for ‘C2’