eval' (ParAnd p q) k u = eval p (\b-> if b then eval q k u else k b) unknown
  where
    unknown pos = eval q (\b-> if b then u pos else k b) (\_-> u pos)
eval' (ParAnd p q) k u = eval p (\b-> if b then eval q k u else k b) unknown
  where
    unknown pos = eval q (\b-> if b then u pos else k b) (\_-> u pos)
eval' (ParAnd p q) k u = eval p (\b-> if b then eval q k u else k b) unknown
  where
    unknown pos = eval q (\b-> if b then u pos else k b) (\_-> u pos)