simplify(plus(A,B),CV,Vars):-simplify(A,AV,Vars),simplify(B,BV,Vars),CV is plus(AV,BV).
simplify(minus(A,B),CV,Vars):-simplify(A,AV,Vars),simplify(B,BV,Vars),CV is minus(AV,BV).
simplify(times(A,B),CV,Vars):-simplify(A,AV,Vars),simplify(B,BV,Vars),CV is times(AV,BV).
simplify(Num,Num,Vars):-number(Num).
simplify(Var,Value,Vars):-atom(Var),member(Var/Value,Vars).