DTD from extended RNC

I would like to use DocBook for a mathematics book.  For that, I need to
extend the DocBook 5.1 schema, to incorporate statements like
"Definition", "Lemma", "Proposition", "Theorem", and "Corollary".  I
would like both RELAX NG and DTD versions of the extended schema; the
DTD is necessary for the PSGML mode in Emacs.

I have made a first attempt at extending the RNC.  The resulting schema,
which I've called `mono.rnc' is deemed valid by `jing':

$ jing -c mono.rnc && echo "VALID"
VALID

I am also able to convert the RNC to an RNG with trang:

$ trang mono.rnc mono.rng && echo "SUCCESS"
SUCCESS

Now, to use PSGML, I have to convert the RNC to a DTD.  However, when I
try to do that with `trang', the command aborts with several errors:

$ trang mono.rnc mono.dtd

/home/raghu/xml/xml/mono.rnc:14:1: error: sorry, combining definitions
with combine="choice" is not supported

/pkg/docbook/rng/docbookxi.rnc:264:6: error: sorry, ambiguous attribute
choice not handled (attribute "type" from namespace
"http://www.w3.org/1999/xlink")

[...]

I have enclosed the schema `mono.rnc' below.  I would be grateful if you
have any suggestions.  I have asked this question in an ongoing thread
at comp.text.xml

https://groups.google.com/d/msg/comp.text.xml/SYhl-SKcKO0/qpVJIKbCBQAJ

but am posting it here, because this is the canonical place for the
question.

Thanks,
Raghu.

---------- mono.rnc ----------

# The namespace for all unqualified names.  We use the prefix `db' for
# it.

default namespace db = "http://docbook.org/ns/docbook"

## Include the DocBook schema which allows XInclude.

include "/pkg/docbook/rng/docbookxi.rnc"

## Add a new choice pattern `db.statement.blocks' to the pattern
## `db.formal.blocks'.  This makes the `db.statement.blocks' pattern a
## valid option in any context where `db.formal.blocks' appears.

db.formal.blocks |= db.statement.blocks

## The `db.statement.blocks' pattern is itself a choice pattern, whose
## values are the patterns that match definitions, theorems, etc.

db.statement.blocks =
  db.corollary
| db.definition
| db.lemma
| db.notation
| db.observation
| db.proposition
| db.theorem

## Here is the definition of the individual values of the
## `db.statement.blocks' choice pattern.

div {
  db.corollary   = element corollary   { db._statement.content }

  db.definition  = element definition  { db._statement.content }

  db.lemma       = element lemma       { db._statement.content }

  db.notation    = element notation    { db._statement.content }

  db.observation = element observation { db._statement.content }

  db.proposition = element proposition { db._statement.content }

  db.theorem     = element theorem     { db._statement.content }
}

## The content of an individual value of the `db.statement.blocks'
## choice pattern.  Adapted from `db.example'.

div {
  db._statement.content =
    db._statement.attlist
  , db._statement.info
  , db.all.blocks +
  , db.caption ?

  db._statement.attlist =
    db._statement.role.attribute ?
  & db.common.attributes
  & db.common.linking.attributes
  & db._statement.label.attribute ?
  & db._statement.floatstyle.attribute ?
  & ( db._statement.width.attribute | db._statement.pgwide.attribute ) ?

  db._statement.info                 = db._info.title.onlyreq

  db._statement.role.attribute       = attribute role { text }

  db._statement.label.attribute      = db.label.attribute

  db._statement.floatstyle.attribute = db.floatstyle.attribute

  db._statement.width.attribute      = db.width.characters.attribute

  db._statement.pgwide.attribute     = db.pgwide.attribute
}

---------- mono.rnc ends here ----------