  We show that in order to decide whether a given probability measure is
laminate it is enough to verify Jensen's inequality in the class of extremal
non-negative rank-one convex integrands. We also identify a subclass of these
extremal integrands, consisting of truncated minors, thus proving a conjecture
made by \v{S}ver\'ak in (Arch. Ration. Mech. Anal. 119 293-300, 1992).
