ifmerge is based on txtmerge written by Mark Podlipec, part of XANIM distribution. Based on infos Royal Frazier's GIF89a Animation Page I improved txtmerge to gifmerge.
gifmerge uses GIF89a extensions, as well the recent developed application extensions supported by Netscape-2.0 with n-loops. For details on GIF89a see at the GIF89a Specifications.