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