Consider the following definite Horn$-$clause KB:

AAxAAyAAz Parent $(x,y)$&Parent$(x,z)$&$yz=>$ Sibling $(y,z)$

AAxAAyAAz Sibling$((v,w)$& Parent $(w,u)$& Male$($u$)=>$ Nephew$($u$,$v$)$

Parent$($Bob$,$Mary$),$ Male$($Bob$),$ Female$($Mary$),$ Parent$($Bob$,$Fred$),$ Male$($Fred$),$ Parent$($Mary$,$Tom$),$

Male$($Tom$),$ Parent$($Mary$,$Ann$),$ Female$($Ann$)$

Assume backward$-$chaining rule$-$based inference is used to try to answer the query: Nephew$($s$,$Fred$).$