q:-halt.
:-style_check(-singleton).

 % I married a widow who had a grown-up daughter.
male(me). female(widow). female(daughter).
parent(widow,daughter).
married(me, widow).  
 %  So I became the stepfather of her daughter.
parent(me,daughter).

 % Some months later, my wife gave birth to a son.
parent(widow, son).
parent(me, son).  male(son).

 % My father, who visited us halte often, fell in love with my step-daughter,
 % and married her.
male(father). parent(father, me).

 % Hence, my father became my son-in-law, and my stepdaughter became my mother.
parent(daughter, me). parent(me, father).

 % The wife of my father, that is my step-daughter, also had a son.
parent(daughter, anotherson).  parent(father,  anotherson).  male(anotherson).

 % My son became became my brother-in-law.
 % of my father as well as my uncle.
 % and I got a brother and at the same time a grandson. 
:-consult('family.plg').
 % My wife is my grandmother,
 % since she is my mother's mother.
 % Hence, I am my wife's husband and at the same time her step-grandson;
 % in other words, I am my own grandfather.
:-nl,write('Try grandfather(me, me)'),nl.