residue was contraband. Testing for certainty’s sake will not, by itself, undermine an otherwise valid probable cause determination. Cf. United States v. Prandy-Binett, 995 F.2d 1069, 1073 (D.C.Cir.1993) ("If [the defendant's] perfume bag held clear ziplock bags containing white powder, the detectives also would not have been sure whether he possess t court erred in admitting the evidence discovered subsequent to the field test at the Buchanan residence. Having held that the evidence was lawfully admitted, we need not address this argument. 8 . We distinguish this case from United States v. Pofahl, 990 F.2d 1456 (5th Cir.), cert. denied, — U.S. -, 114 S.Ct. 266, 126 L.Ed.2d 218 (1993). In Pofahl, we held that .2d 433 (1994); United States v. Thomas, 932 F.2d 1085, 1090 (5th Cir.1991) (<HOLDING>), cert. denied, 502 U.S. 1038, 112 S.Ct. 887,
holding that the cocaine guidelines  are advisory only holding that the sentencing guidelines disparate treatment of crack cocaine is not racially discriminatory under the fifth amendments equal protection guarantees holding that the statute and sentencing guidelines applicable to crack cocaine have a reasonable basis and are not vague under commonly understood usages holding that district court did not err in determining that cocaine base was crack cocaine based on chemical analysis identifying cocaine base together with competent lay testimony bridging the evidentiary gap between cocaine base and crack cocaine and refusing to require showing of smokeability ie water solubility or melting point for purposes of establishing crack cocaine under the guidelines since smokeability distinguishes cocaine base from powder cocaine not from crack holding that sentencing disparity between crack and powder cocaine is constitutional