title: Nephew of my own father in prolog
body: <p>i'm programming a family in prolog and i'm having trouble with the nephew implementation. When I ask if erick is the nephew of Alberto it returns true, when it should return false because Alberto is the father of erick, however, it does works for all the other cases that should be true. If someone could help me I would be very grateful.
  My code:</p>
  
  <pre><code>man(beto).
  man(fransisco).
  man(alberto).
  man(jaime).
  man(manolo).
  man(nolo).
  man(lito).
  man(manuel).
  man(erick).
  man(jesu).
  man(jesus).
  woman(emi).
  woman(harumi).
  woman(haru).
  woman(yuneisi).
  woman(yasmeli).
  woman(mioara).
  woman(elia).
  woman(iza).
  woman(alice).
  woman(ofelia).
  woman(arlet).
  parent(manuel, alberto).
  parent(ofelia, alberto).
  parent(manuel, jaime).
  parent(ofelia, jaime).
  parent(manuel, manolo).
  parent(ofelia, manolo).
  parent(alberto, erick).
  parent(alberto, beto).
  parent(alberto, fransisco).
  parent(emi, erick).
  parent(emi, beto).
  parent(manolo, nolo).
  parent(manolo, arlet).
  parent(nolo, lito).
  parent(iza, lito).
  parent(mioara, yuneisi).
  parent(mioara, yasmeli).
  parent(jaime, yuneisi).
  parent(jaime, yasmeli).
  parent(jesus_padre, jesu)
  parent(jesus_padre, alice).
  parent(jesus_padre, haru).
  parent(harumi, haru).
  parent(harumi, jesu).
  parent(harumi, alice).
  father(X,Y) :- parent(X,Y), man(X).
  mother(X,Y) :- parent(X,Y), woman(X).
  brother(X,Y) :- man(X), parent(F, X), parent(F, Y).
  sister(X,Y) :- woman(X), parent(P, X), parent(P, Y).
  grandpa(X,Y) :- father(F,Y), father(X,F), man(X).
  grandma(X,Y) :- father(F,Y), mother(X,F), woman(X).
  son(X,Y) :- father(Y,X), man(X).
  nephew(X,Y) :- father(F,X), brother(F,Y).
  </code></pre>