Although many algorithms have been published for the problem of constructing the convex hull of a simple polygon, nearly half of them are incorrect.[3] McCallum and Avis provided the first correct algorithm.[4] A later simplification by Graham & Yao (1983) and Lee (1983) uses only a single stack data structure. Their algorithm traverses the polygon clockwise, starting from its leftmost vertex. As it does, it stores a convex sequence of vertices on the stack, the ones that have not yet been identified as being within pockets.