I came to the idea that variable binding could be used to define
namespace prefix mapping in the context of an expression, using
"pseudo-variables" in the same way as pseudo-attributes are used for
namespace binding, as illustrated below:

for
 $xmlns in 'http://default/uri',
 $xmlns:c in 'http://example.org/customer',
 $xmlns:p in 'http://example.org/personal-info'
return
 /data/c:customer/p:name

Please give me your feedback:
- Is it allowed?
- Is it semantically compatible with XPath2 / XML Namespaces?
- Is is usable for human authors?

I'd like to solicit reactions to a proposal to define an informal standard
for binding namespaces in strings containing XPath expressions.