RelaxNG: Having more than one of a similar element in <interleave>

The requirement is as such:

A feed will have multiple links
A "B" type link MUST appear atleast once
An "A" type link MAY appear zero or more times
"A" type links can appear in other places too hence need to keep the define independent

I get the following error with the code
lxml.etree.RelaxNGParseError: Element or text conflicts in interleave, line 6

Example code below

from io import StringIO
from lxml import etree

XML = StringIO('''
<root xmlns="http://www.w3.org/2005/Atom">
    <link rel="a1" />
    <link rel="b1" />
    <link rel="a2" />
    <link rel="b2" />
</root>
''')

RNG = StringIO('''
<grammar xmlns="http://relaxng.org/ns/structure/1.0">

    <start>
        <element name="root" ns="http://www.w3.org/2005/Atom">
            <interleave>
                <zeroOrMore>
                    <ref name="aLink"/>
                </zeroOrMore>
                <oneOrMore>
                    <ref name="bLink"/>
                </oneOrMore>
            </interleave>
        </element>
    </start>

    <define name="aLink" ns="http://www.w3.org/2005/Atom">
        <element name="link">
            <choice>
                <attribute name="rel">
                    <value>a1</value>
                </attribute>
                <attribute name="rel">
                    <value>a2</value>
                </attribute>
            </choice>
        </element>
    </define>

    <define name="bLink" ns="http://www.w3.org/2005/Atom">
        <element name="link">
            <choice>
                <attribute name="rel">
                    <value>b1</value>
                </attribute>
                <attribute name="rel">
                    <value>b2</value>
                </attribute>
            </choice>
        </element>
    </define>

</grammar>
''')

rng_tree = etree.parse(RNG)
rng_schema = etree.RelaxNG(rng_tree)

rng_schema.validate(XML)

Solution

I fear that it's a typical case that is forbidden by the specification of Relax NG

7.4. Restrictions on interleave

For a pattern p1 p2 ,

there must not be a name that belongs to both the name class of an element pattern referenced by a ref pattern occurring in p1 and the name class of an element pattern referenced by a ref pattern occurring in p2, and a text pattern must not occur in both p1 and p2.

https://www.oasis-open.org/committees/relax-ng/spec-20011203.html#interleave-restrictions