factr(0,1).
factr(N,F):- N_1 is N - 1, factr(N_1,F_1), F is N * F_1.

facti(N,F):-facti(0,1,N,F).
facti(N,F,N,F).
facti(I,Fi,N,F):- J is I+1, Fj is J*Fi, facti(J,Fj,N,F).

sum_to(1,1).
sum_to(N,S):-N1 is N-1, sum_to(N1,S1), S is S1+N. 

sum_toc(N,1):-N=<1,!.
sum_toc(N,S):-N1 is N-1, sum_toc(N1,S1), S is S1+N. 

/*
sum_to(2,3):- 1 is 2-1, sum_to(1,1), 3 is 1+2. 
sum_to(1,1). S1<-1
sum_to(1,S1):-0 is 1-1, sum_to(0,S1p), S1 is S1p+1. 
*/

weird(1).

parent(john,mary).
parent(mary,mary).
sibling(X,Y):-not(X=Y),parent(X,P),parent(Y,P).
siblingx(X,Y):-notx(X=Y),parent(X,P),parent(Y,P).

notx(P):-call(P),!,fail.
notx(P).
/*
X = if A then B else C

X:-A,!,B.
X:-C.
*/