What is the difference between a detainer and a writ of habeas corpus ad prosequendum?

Because we uphold appellant's claims under the Detainers Act, we need not discuss his other claims. ⁴ Many of the questions treated by the government here as open were recently settled in this circuit by our decision in United States v. Mauro, 544 F.2d 588 (2d Cir. 1976) where we held that under the Detainers Act the United States is bound by the statute's definition of it as both a sending and a receiving "State" and that the writ of habeas corpus ad prosequendum constitutes a "detainer" and a "request" by a prosecuting authority within the meaning of the Act, with the present author dissenting in that case from the majority's holding that a writ of habeas corpus ad prosequendum constitutes a "detainer." The government here, however, has conceded that appellant in this case was subject to a detainer, separate and apart from the writ, filed by it with the Massachusetts authorities. ⁵ Whether the writ independently constitutes a detainer, therefore, is not at issue here.