Please help me understand maps

Hi Folks,

My understanding of a map is that it is an object containing a bunch of
key/value pairs.

That seems pretty simple.

In the XSLT 3.0 specification it has this example:

<xsl:variable name="isbn-index" as="map(xs:string, element(book))"

Okay, $isbn-index is that name of a map. The keys for that map must be strings
and the value associated to each key must be a <book> element.

Got it.

Then the example assigns $isbn-index a value:

    select="map:new(for $b in //book return map{$b/isbn := $b})"/>

Yikes!

What is that?

Let me try to parse it:

Step 1. Create a new map. That's what "map:new(...)" does.

Step 2. Loop through each <book> element. That's what "for $b in //book return
..." does.

Step 3. For each <book> element create a map. That's what "map{$b/isbn := $b}"
does. Huh?

Step 1 creates a new map, so it should be assigned a bunch of key/value pairs,
right? Step 3 is blasting it with a bunch of maps. Huh? Where are the new
map's key/value pairs? I don't get it. Please help.

Oh! Perhaps since each "inner map" object contains just one key/value pair,
each inner map object gets "unwrapped" to yield its key/value pair to the
"outer map" object. Ha! How do you like that theory?

/Roger