Duration of anticoagulant therapy after a first episode of an unprovoked pulmonary embolus or deep vein thrombosis: guidance from the SSC of the ISTH - Baglin, T; Bauer, K; Douketis, J; Buller, H; Srivastava, A; Johnson, G; SSC of the ISTH (J Thromb Haemost, 2012)