Gather a xor b
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We produce a world composite image by collecting messages from six trollflow2 instances, corresponding to production of imagery from six geostationary satellite sensors: MTG-FCI, MSG-SEVIRI (IODC), GK2A-AMI, HIMAWARI-AHI, GOES-West ABI, and GOES-East ABI. At present, we ignore GK2A-AMI and produce a world composite image when the other five constituent images have been produced. This is suboptimal, because if HIMAWARI-AHI is missing or delayed, GK2A-AMI should be able to jump in. It would be nice to instruct the segment gatherer to collect either HIMAWARI-AHI or GK2A-AMI, whichever comes first, and then drop the other one.
Another use case is when one satellite is a backup of another. For example, when MTG-FCI is not available, fall back to MSG-SEVIRI as a drop-in replacement.