Expression

class Constant\4
    attr a1
    meth init(X) a1:= X end
    meth evaluate($) @a1 end
end
class Variable\4
    attr a1
    meth init(X) a1:= X end
    meth set(X) a1:=X end
    meth evaluate($) @a1 end
end
class Addition\4
    attr a1 a2
    meth init(X Y) a1:=X a2:=Y end
    meth evaluate($)
      {@a1 evaluate($)} + {@a2 evaluate($)}
    end

end
class Subtraction\4
    attr a1 a2
    meth init(X Y) a1:=X a2:=Y end
    meth evaluate($)
      {@a1 evaluate($)} - {@a2 evaluate($)}
    end
end
class Multiplication\4
    attr a1 a2
    meth init(X Y) a1:=X a2:=Y end
    meth evaluate($)
       {@a1 evaluate($)} * {@a2 evaluate($)}
    end
end
class Division\4
    attr a1 a2
    meth init(X Y) a1:=X a2:=Y end
    meth evaluate($)
        {@a1 evaluate($)} div {@a2 evaluate($)}
    end
end