class Constant
    attr a1
    meth init(X) a1:=X end
    meth evaluate($) @a1 end
end
class Variable
    attr a1
    meth init(X) a1:=X end
    meth set(Y) a1:=Y end
    meth evaluate($)  @a1 end
end
class Addition
    attr a1 a2
    meth init(VarX VarY) a1:=VarX a2:=VarY end
    meth evaluate($)
        {@a1 evaluate($)}+{@a2 evaluate($)} end
end
class Subtraction
    attr a1 a2
    meth init(VarX VarY) a1:=VarX a2:=VarY end
    meth evaluate($)
        {@a1 evaluate($)}-{@a2 evaluate($)} end
end
class Multiplication
    attr a1 a2
    meth init(VarX VarY) a1:=VarX a2:=VarY end
    meth evaluate($)
        {@a1 evaluate($)}*{@a2 evaluate($)} end
end
class Division
    attr a1 a2
    meth init(VarX VarY) a1:=VarX a2:=VarY end
    meth evaluate($)
        {@a1 evaluate($)} div {@a2 evaluate($)} end
end