make a format available to the loader. Returns a function that removes it again, which is what
a test uses to put the registry back as it found it; production code registers once, at import.
Registering a name twice replaces the earlier format, so a caller can override a built-in.
make a format available to the loader. Returns a function that removes it again, which is what a test uses to put the registry back as it found it; production code registers once, at import.
Registering a name twice replaces the earlier format, so a caller can override a built-in.