====== Opis ======

Predikat ''sister(X, Y)'' velja natanko takrat, kadar je ''X'' sestra od ''Y''. Primer klica:

  ?- sister(X, patricia).
    X = vanessa.

===== Predznanje =====

{{:codeq:prolog:naloge:famrel_graph.gif?direct|}}

Povezave so podane s predikatom ''parent/2''. Predikata ''male/1'' in ''female/1'' določata spol vsake osebe (vozlišča).

  parent(tina,william).                      male(william).
  parent(thomas,william).                    male(thomas).
  parent(thomas,sally).                      male(jeffrey).
  parent(thomas,jeffrey).                    male(andrew).
  parent(sally,andrew).                      male(steve).
  parent(sally,melanie).                     male(patrick).
  parent(andrew,joanne).                     male(john).
  parent(jill,joanne).                       male(michael).
  parent(joanne,steve).                      male(frank).
  parent(william,vanessa).                   male(george).
  parent(william,patricia).                  male(morty).
  parent(vanessa,susan).                     male(jerry).
  parent(patrick,susan).                     male(kramer).
  parent(patricia,john).                     male(aleksandr).
  parent(john,michael).                      male(alessandro).
  parent(john,michelle).                     male(aleksander).
  
  parent(frank,george).                      female(tina).
  parent(estelle,george).                    female(sally).
  parent(morty,jerry).                       female(melanie).
  parent(helen,jerry).                       female(joanne).
  parent(jerry,anna).                        female(jill).
  parent(elaine,anna).                       female(vanessa).
  parent(elaine,kramer).                     female(patricia).
  parent(george,kramer).                     female(susan).
                                             female(michelle).
  parent(margaret,nevia).                    female(estelle).
  parent(margaret,alessandro).               female(helen).
  parent(ana,aleksander).                    female(elaine).
  parent(aleksandr,aleksander).              female(anna).
  parent(nevia,luana).                       female(margaret).
  parent(aleksander,luana).                  female(ana).
  parent(nevia,daniela).                     female(nevia).
  parent(aleksander,daniela).                female(luana).
                                             female(daniela).

====== Rešitev ======

  sister(X, Y) :-
      parent(Z, X),
      parent(Z, Y),
      female(X),
      X \== Y.

====== Namigi ======

^ # ^ Namig                                                     ^
| 1 | ''X'' in ''Y'' morata imeti (vsaj enega) skupnega starša. |
| 2 | ''X'' mora biti ženskega spola.                           |
| 3 | ''X'' ne more biti sestra sama sebi.                      |
| 4 | ''Y'' ni nujno ženskega spola / relacija ni simetrična.   |

{{:codeq:prolog:naloge:sister-hints.jpg?direct|}}

====== Testni primeri ======

  ex(sister(sally, william)).        nex(sister(william, jeffrey)).
  ex(sister(melanie, andrew)).       nex(sister(tina, thomas)).
  ex(sister(luana, daniela)).        nex(sister(nevia, aleksander)).
  ex(sister(daniela, luana)).        nex(sister(joanne, jill)).
  
===== #1 =====

  nex(sister(vanessa, andrew)).
  nex(sister(elaine, george)).
  nex(sister(sally, thomas)).

===== #2 =====

  nex(sister(andrew, melanie)).
  nex(sister(patrick, vanessa)).
  nex(sister(michael, michelle)).
  
===== #3 =====

  nex(sister(vanessa, vanessa)).
  nex(sister(sally, sally)).
  nex(sister(daniela, daniela)).