expand-sequence/expand-group: Retain until hoisting

This is a rather small change for quite a bit of effort in researching what
was going wrong.  It's at last seven rabbit holes deep, or maybe several
herd of yaks, depending on your choice of measure and the current conversion
rate.

The problem can be summarized fair succinctly:
`expand-sequence/expand-group` exists to prevent an expansion repass for
every single child element of the `expand-sequence`, which would be
quadratic.  Basically, it restores the usual template expansion process for
that set of children.

But apparently `expand-group` was stripped on the first pass, which expanded
its children inline, which then meant that each of the children were subject
to their own individual passes, defeating the purpose of the optimization.

As is the nature of quadratic-time processes, that was not noticed until
inputs became especially large, and not only that, but were combined with
nested `expand-sequence`s.

I would say that this never worked the way that I intended it to, but I'm
not certain.  I was working quite a bit with TeX at the time, so it's
possible that I modeled it after `\expandafter`.  But that's not an
appropriate model for TAME.

TAMER will be removing expand-sequence entirely, since it will have enough
of an understanding of the source system to determine what requires
expansion and what requires ordering (e.g. for symbol table
iteration).  I'll also be making changes to simplify the process by further
restricting what type of iteration can take place.  But for the time being,
the change was necessary.

In our largest systems, this change cut off ~15m total of build time if run
serially (at `-j1`).  After sorting two runtabs for comparison (e.g. `sort
-k4`), you can get the total like so:

   $ paste <( sort -k4 runtab-a ) <( sort -k4 runtab-b )
       | grep xmlo\$ \
       | cut -f2,5,6 \
       | awk '{ total += ($1 - $2) } END { print total / 1000 }'

Similarly, this Awk expression will give the time differences:

   $ awk '{ print ($1 - $2)/1000, $5 }'

Further, the previous commit also introduced a `xmle-sym-cmp` tool to check
for differences between xmle symbol tables in an automated way, irrespective
of ordering (since there are many valid topological sorts).  This revealed
that the change fixed a bug (likely because of the forced repass after
`expand-group` hoisting) that was causing symbol table introspection to fail
to discover symbols in certain cases, which in our case, was resulting in
the failure to generate a small number of aggregate classifications
correctly.  The whole repass system is a concerning mess, but it's not worth
the effort to try to redo all of that when that work can be done in TAMER.

DEV-15069

src/current/include/preproc/template.xsl

@@ -1523,8 +1523,21 @@
 -->
 <template mode="preproc:macros" priority="5"
               match="lv:expand-group">
-  <!-- strip expand-group -->
+  <!-- the expand-group node will be stripped during hoisting -->
-  <apply-templates mode="preproc:macros" />
+  <copy>
+    <apply-templates mode="preproc:macros" />
+  </copy>
+</template>
+
+
+<!--
+  expand-group must be unwrapped when hositing out of an expand-sequence.
+-->
+<template mode="eseq:hoist" priority="5"
+          match="lv:expand-group">
+  <!-- unwrap children -->
+  <sequence select="node()" />
+  <preproc:repass need-sym="(lv:expand-group hoist)" />
 </template>
 
 
@@ -1564,7 +1577,6 @@
 </function>
 
 
-
 <template mode="preproc:macros" priority="9"
               match="node()[ not( . instance of element() ) ]">
   <sequence select="." />

src/preproc/expand/expand-sequence.xsl

@@ -412,6 +412,9 @@
 
   If no head node exists, the result is the single expansion sequence
   node unchanged.
+
+  Hositing behavior may be configured via the @code{eseq:hoist}
+  template mode.
 -->
 <function name="_eseq:hoist" as="node()+">
   <param name="eseq-node" as="element()" />
@@ -419,7 +422,7 @@
   <variable name="head" as="node()?"
             select="$eseq-node/node()[1]" />
 
-  <sequence select="$head" />
+  <apply-templates mode="eseq:hoist" select="$head" />
 
   <!-- This @code{for-each} is purely to set the context for
        @code{copy}, since we do not know the sequence element
@@ -432,4 +435,14 @@
   </for-each>
 </function>
 
+<!--
+  Hoist a node out of a sequence.
+
+  The caller may provide templates that alter the behavior of hositing.
+-->
+<template mode="eseq:hoist" priority="1"
+          match="node()">
+  <sequence select="." />
+</template>
+
 </stylesheet>