holiday(wednesday, december25).
weather(wednesday, fair).
weather(saturday,fair).
weather(sunday,fair).
weekend(saturday).
weekend(sunday).
picnic(Day) :- !, weather(Day,fair), weekend(Day).
picnic(Day) :- holiday(Day,december25).

?picnic(When)
When = saturday ?
When = sunday

;why do saturday and sunday pass? assuming the first value the backtracker uses
;is wednesday, it should be locked in from the cut, and then fail on weekend(wednesday), giving no valid
; solutions?