I'm using SWI-Prolog on Windows and am getting the following error:
14 ?- parent(X, Y) :- child(Y, X).
ERROR: toplevel: Undefined procedure: (:-)/2 (DWIM could not correct)
I'm not entirely sure what's going on, as this worked last week and I am just starting to learn Prolog.
The FAQ says it all: http://www.swi-prolog.org/FAQ/ToplevelMode.html
You need to create a file and write your program with rules there. The top level command line will only allow you to issue queries.