The package sgml-common (sgml-data on Debian) includes non-Free files in the sgml-iso-entities directory under the following licence:
'Permission to copy in any form is granted for use with conforming SGML systems and applications as defined in ISO 8879, provided this notice is included in all copies.' Files under the same licence are also included in linuxdoc-tools in the directories iso-entities/entities/ and entity-map/sdata/ (for example ISOtech). Furthermore, files in the directory classpath/tools/resource/gnu/classpath/tools/gjdoc/dtd/ent/ in GCC are based on these files. The SGML entity files are needed to build documentation for several projects. It seems like it would be difficult to write the SGML entity files much differently, so it is possible that they are not copyrightable. Also, since Debian includes the files it is possible there has been licence clarification or such at some point.
