swi-prolog how to prevent repeated output -


i have knowledge base

parent(dad, son). parent(mom, son). parent(dad, daughter). parent(mom, daughter).

i have functions 

mother/2 father/2 child/2

that work expected, when use sibling

sibling(x,y):-     parent(z,x),     parent(z,y),     not(x=y).

the output repeats

?- sibling(x,y). x = son, y = daughter ; x = daughter, y = son ; x = son, y = daughter ; x = daughter, y = son ;

i understand why it's repeating, question how limit 1 pair non-repeating? in

?- sibling(x,y). x = son, y = daughter ;

you try lurker wrote : using x @< y instead of not(x = y), you'll :

?- sibling(x,y). x = daughter , y = son; x = daughter , y = son;

and sibling(son,daughter). return no whereas should return true.

or can hit enter after getting first result , not prompted other results (but depends on environment use prompt every result without asking if want them or not).

edit:

what can limit results following :

sibling(x,y):-     parent(z,x),     parent(z,y),     parent(a,x),     parent(a,y),     not(x=y),     z @< a.

let me explain : in current implementation 4 results because

  • son & daughter have same mom
  • daughter & son have same mom
  • son & daughter have same dad
  • daughter & son have same dad

with previous implementation, sibling(x,y) true, x , y need have same 2 parents, you'll :

  • son & daughter have same 2 parents
  • daughter & son have same 2 parents

and knowledge minimum 2 results can have if want both sibling(son,daughter) & sibling(daughter,son) true.


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