union: Pass through map 0, so that if ignore is set, the input map will still be output. Allow either map's input label to contain the other's input label to enable e.g. appending mappings for an older input version to those for a newer input version.
union: Pass through map 0, so that if ignore is set, the input map will still be output. Allow either map's input label to contain the other's input label to enable e.g. appending mappings for an older input version to those for a newer input version.