I received the following report by Martin Vidner <mvidner-at-suse de> as
https://bugzilla.novell.com/show_bug.cgi?id=132722 and I'm able to reproduce it
with libxml2-2.6.22.  Here I repeat Martin's text literally:

The attached set of Relax NG schemas fails to compile in xmllint. Another
validator, jing, has no such problem. (I test on a null document for
simplicity).

mvidner@valkyrie:bad2$ xmllint --noout --relaxng suse-primary.rng /dev/null
suse-primary.rng:3: element include: Relax-NG parser error : xmlRelaxNG:
include primary.rng has a start but not the included grammar
suse-primary.rng:5: element ref: Relax-NG parser error : Some <start> element
miss the combine attribute
primary.rng:6: element ref: Relax-NG parser error : Found forbidden pattern
start//interleave
Relax-NG schema suse-primary.rng failed to compile
/dev/null:1: parser error : Document is empty

^
/dev/null:1: parser error : Start tag expected, '<' not found

^
mvidner@valkyrie:bad2$ jing suse-primary.rng /dev/null
/dev/null:1: fatal: Document root element is missing.

Here are 3 rnc files and 3 rng equivalents generated by "trang suse-primary.rnc
suse-primary.rng".
Note that in the inclusion chain (suse-primary .. primary .. rpm-ns) the start
element is overriden each time. If the override is commented out in
suse-primary, libxml2 accepts the schema.
(The override is actually useless here but not in the original files that were
simplified for this testcase)

From the error messages I infer that libxml2 wrongly applies the "combine"
method, instead of replacing the definitions entirely as required when doing an
include. (See 9.4 Replacing definitions in the RNC Tutorial,
http://relaxng.org/compact-tutorial-20030326.html#id2815849)