Who can advise, please, about an error message Nat1: Maybe.fromJust: Nothing ? Probably, this is a bug in my program, and first, I need to locate it. The head test module is Nat1.hs. The output is -------------------------------------------------------------------- ... ... --(ProofSearchState ---------------------------------- tree = (formula1 Any [(1, NegCp (spent cost 1283), (formula1 Any [])) (2, ArC fromList [(n,n_c)], (formula6 Any [(1, NegCp (spent cost 683), (formula6 Any []))])) (3, Ind n, (formula1 [(1, IndMember, Nat1: Maybe.fromJust: Nothing ----------------------------------------------------------------------- -- and break. By the last line GHC says, probably, that executing the module Nat1 caused the call of `fromJust Nothing'. And Nat1.hs does not contain any call of fromJust, neither even the substring of "fromJust". Nat1 imports many other modules. Why GHC does not mention some other module from which this `fromJust Nothing' has directly appeared? Thank you in advance for your explanations, ----------------- Serge Mechveliani mechvel@botik.ru