lause within clause 8.2.5 (which defines a process that is invoked only when nal ref idc is not equal to 0), this process may also be invoked when nal ref idc is equal to 0 (as specified in clause 8). The reasons for the location of this subclause within the structure of this Recommendation | International Standard are historical. NOTE 2 - This process can only be invoked for a conforming bitstream when gaps_in_frame_num_value_allowed_flag is equal to 1. When gaps_in_frame_num_value_allowed_flag is equal to 0 and frame num is not equal to PrevRefFrameNum and is not equal to PrevRefFrameNum + 1) % MaxFrameNum, the decoding process should infer an unintentional loss of pictures. When this process is invoked, a set of values of frame num pertaining to "non-existing" pictures is derived as all values taken on by UnusedShortTermFrameNum in Equation 7-23 except the value of frame_num for the current picture. For each of the values of frame_num pertaining to "non-existing" pictures, in the order in which the values of UnusedShortTermFrameNum are generated by Equation 7-23, the following ordered steps are specified: 1. The decoding process for picture numbers as specified in clause 8.2.4.1 is invoked. 2. The sliding window decoded reference picture marking process as specified in clause 8.2.5.3 is invoked. The decoding process generates a frame and the generated frame is marked as "non-existing" and "used for short-term reference". The sample values of the generated frame may be set to any value. The following constraints shall be obeyed: a) The bitstream shall not contain data that result in the derivation of a co-located picture colPic that is marked as "non-existing" in any invocation of the derivation process for the co-located 4x4 sub-macroblock partitions specified in clause 8.4.1.2.1. b) The bitstream shall not contain data that result in the derivation of a reference picture that is marked as "non-existing" in any invocation of the reference picture selection process specified in clause 8.4.2.1. The bitstream sha