Unexpected behaviour when replacing start symbol of included grammar

[viliaapro]

Gentle people,
I've found an unexpected behaviour in both Jing and Emacs's RNG-validator; the operators |= and &= work just like = when used on the start symbol of an included grammar.
Given three files, foo.rnc, bar.rnc and foo.xml, the command jing -c foo.rnc foo.xml succeeds but jing -c bar.rnc foo.xml fails.

<!-- foo.xml -->
<foo/>

# foo.rnc
start	= foo
foo	= element foo {empty}

# bar.rnc
bar = element bar {empty}

start =
    grammar {
    include "foo.rnc"
	{ start |= parent bar }
    }

Replacing |= with &= produces the same error: it is as if there was as replacement start = parent bar.

Best regards and thanks,
Vili Aapro