• Source 
    1. simplify(plus(A,B),CV,Vars):-simplify(A,AV,Vars),simplify(B,BV,Vars), CV is AV+BV.
    2. simplify(minus(A,B),CV,Vars):-simplify(A,AV,Vars),simplify(B,BV,Vars), CV is AV-BV.
    3. simplify(times(A,B),CV,Vars):-simplify(A,AV,Vars),simplify(B,BV,Vars), CV is AV*BV.
    4. simplify(Num,Num,Vars):-number(Num).
    5. simplify(Var,Value,Vars):-atom(Var),member(Var:Value,Vars).