@prefix : <sum.n3#>.  
@prefix log: <http://www.w3.org/2000/10/swap/log#>.
@prefix math: <http://www.w3.org/2000/10/swap/math#>.


         { ?x :hasVal ?a. ?y :hasVal ?a.  (?a ?a) math:sum ?c }
         log:implies 
          {?x :val ?c}.

:b :hasVal 4.
:cd :hasVal 4.
:d :hasVal 4.