Some stages, most notably XMLSignatureSigningStage, contain their own set of constants for the various URIs used in XML DSIG. Worse, these are public fields and therefore part of the API.
There's no good reason for this these days. In Java 11, all of these constants are available as part of the Java API. We should remove our duplicates, and use the Java API's version in our implementation.
Removing the fields will change the API, so should be prioritised for 0.10 so that it can be done pre-1.0.