FlatTissue[vertices, cells] is an uninstantiated function that is used to represent a flattened form of a the Tissue data structure. A Flat Tissue does not explicitly list the edges, since they can be derived from the vertices; this format is not normally used internally because the expansion is required for most operations. However, it is more efficient when storing the tissue externally since the list of edge numbers can be omitted. Each cell is represented by a list of vertex indices (cells).
Tissue2FlatTissue[T] converts a Tissue object T into a FlatTissue data structure.
FlatTissue2Tissue[F] converts a FlatTissue object F into a Tissue data structure.