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