1. A method of storing motion vectors of a first reference frame which are permitted to point to a plurality of further reference frames, the method comprising: selecting one of said further reference frames wherein the further reference frames comprise one or more reference frames in the future and one or more reference frames in the past; mapping to the selected reference frame motion vectors of said first reference frame that point to one of said further reference frames other than the selected further reference frame so that after the mapping said motion vectors of the first reference frame point to the selected reference frame; storing the motion vectors subjected to said mapping; and using the stored motion vectors to encode a sequence of digital images into a bitstream or to decode a bitstream into a sequence of digital images.