[xsl] How to write (existential) predicates with maps/Why is there no effective boolean value for a map?

["Martin Honnen martin.honnen@xxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

When using XPath 3.1 (e.g. in XSLT 3) on maps I have found that I have 
to change my coding habit a bit when writing predicates that want to 
check the existence of some nested map, while I hoped to be able to 
write e.g.

  ?locations?*[?types?*[?name = 'foo']]

to select all (map) members of the "locations" array that have a "types" 
array with at least one (map) member having a property "name" with value 
"foo" I get an error

Effective boolean value is not defined for sequence starting with an 
atomic value other than a boolean, number, or string


So in contrast to my experience with writing predicates on XML it seems 
for maps I have to explicitly use the "exists" function e.g.

  ?locations?*[exists(?types?*[?name = 'foo'])]

or a "some .. in" expression

  ?locations?*[some $m in ?types?* satisfies $m?name = 'foo']


Is there any more compact way to write such a check?

What is the reason that the effective boolean value was not extended to 
give true for a sequence with a map?