Can fold-left process the attributes nodes of a stream

I am struggling to find a compact way to use XSLT 3 streaming and an
accumulator to sum up the attribute values of certain elements,e.g.
xsl:accumulator name="attribute-sums" as="map(xs:QName, xs:decimal)"
initial-value="map{}" streamable="yes" to accumulate the sum of each
attribute by the name of the attribute.

One problem is that accumulators don't fire on attribute nodes, only on
element nodes, therefore I have to write the accumulator-rule to match
on the parent element of the attributes and compute all the sums in a
single rule;

B  fold-left(@*, $value, function($v, $a) { map:put($v, node-name($a),
(map:get($v, node-name($a)), 0)[1] + xs:decimal($a)) })

seemed like a compact and elegant use of XPath 3.1 but using a dynamic
inline function as the argument to fold-left doesn't seem to pass the
streamability analysis.

Then I tried to set up a user-defined function to be passed in as the
third argument to fold-left, but somehow that approach doesn't seem to
work either as a streamable stylesheet function needs to take the
streamable nodes as the first parameter while the signature of the
function $f of fold-left would make the attribute node the second parameter.

So the only options I could think of is not passing the attribute nodes
themselves to fold-left but rather an array or map of the name and value
I need to store in the accumulator e.g.

fold-left(@* ! [node-name(), xs:decimal(.)], $value, function($v, $anv)
{ map:put($v, $anv?1, (map:get($v, $anv?1), 0)[1] + $anv?2) })

The xsl:iterate approach feels too verbose, I kind of think fold-left
should suffice to process the sequence of attributes (without that
escape hatch to map them first to an array or map).