# Pastebin SZklR7W2
parent(bob, nancy).
parent(nancy, dave).
ancestor(X,Y) :- parent(X,Y).
ancestor(X,Z) :- parent(X,Y), ancestor(Y,Z) 

becomes

name : type
bob, nancy : name
parent : name -> name -> type
fact1 : parent bob nancy
fact2 : parent nancy dave
rule1: ancestor X Y <- parent X Y 
rule2: ancestor X Z <- parent X Y <- ancestor Y Z

then you can ask for results and instead of getting

? ancestor(bob,X)
X = nancy
X = dave

you get the derivation

? X : ancestor bob Y
rule1 fact1 : ancestor bob nancy
rule2 fact2 (rule1 fact1) : ancestor bob dave