class Constant\4
    attr x
    meth init(A) x := A end
    meth set(A) x := A end
    meth evaluate($) @x end
end

class Variable\4
    attr y
    meth init(A) y := A end
    meth set(A) y := A end
    meth evaluate($) @y end
end

class Addition\4
    attr a1 a2
    meth init(A B) a1 := A a2 := B end
    meth evaluate($) {@a1 evaluate($)} + {@a2 evaluate($)} end
end

class Subtraction\4
    attr a1 a2
    meth init(A B) a1 := A a2 := B end
    meth evaluate($) {@a1 evaluate($)} - {@a2 evaluate($)} end
end

class Multiplication\4
    attr a1 a2
    meth init(A B) a1 := A a2 := B end
    meth evaluate($) {@a1 evaluate($)} * {@a2 evaluate($)} end
end

class Division\4
    attr a1 a2
    meth init(A B) a1 := A a2 := B end
    meth evaluate($) {@a1 evaluate($)} div {@a2 evaluate($)} end
end