What is the test for establishing that a trip was for the purpose of prostitution?

Appellants' primary contention is that the evidence was insufficient to conclude that the trip was "for the purpose of prostitution." On this element, they both concede that the requisite intent may be proved by circumstantial evidence and that such evidence may include the conduct of the parties both before and within a reasonable time after the transportation. See United States v. Snow, 507 F.2d 22 (CA7 1974); United States v. Tyler, 424 F.2d 510 (CA10 1970), cert. denied 400 U.S. 839, 91 S.Ct. 78, 27 L.Ed.2d 73. See also United States v. Szymanski, 431 F.2d 946 (CA9 1970).