Add graph:make-from-vertices

* src/graph.xsl (graph:make-from-vertices): Added
* test/graph-test.xsl (foo:graph-with-dupes): Added duplicate edges
* test/graph.xspec: Added tests

src/graph.xsl

@@ -29,6 +29,7 @@
 
 <import href="../hoxsl/src/apply.xsl" />
 
+
 <!--
   @node Dependency Graph
   @section Dependency Graph
@@ -51,6 +52,29 @@
     use the node QNames for type checks.}
 -->
 
+
+<!--
+  Create a graph from the given vertex set @var{$vertices}.
+  The resulting graph will be normalized with duplicate vertices and
+    edges removed,
+    making it suitable for ad hoc graph generation.@c
+  @footnote{This is done by calling @ref{graph:union}.}
+-->
+<function name="graph:make-from-vertices"
+          as="element( preproc:sym-deps )">
+  <param name="vertices" as="element( preproc:sym-dep )*" />
+
+  <variable name="graph" as="element( preproc:sym-deps )">
+    <preproc:sym-deps>
+        <sequence select="$vertices" />
+    </preproc:sym-deps>
+  </variable>
+
+  <!-- dedupe/normalize -->
+  <sequence select="graph:union( $graph )" />
+</function>
+
+
 <!--
   Produce a new graph that is the transpose of
     @var{$graph}@mdash{}that is,

test/graph-test.xsl

@@ -146,9 +146,9 @@
 <variable name="foo:graph-vtwo-vthree" as="element( preproc:sym-deps )">
   <preproc:sym-deps>
     <preproc:sym-dep name="a">
-      <preproc:sym-ref name="a" attr1="foo" />
       <preproc:sym-ref name="b" attr1="foo" attr2="bar" />
       <preproc:sym-ref name="c" />
+      <preproc:sym-ref name="a" attr1="foo" />
     </preproc:sym-dep>
 
     <preproc:sym-dep name="b">

test/graph.xspec

@@ -29,6 +29,42 @@
              stylesheet="graph-test.xsl">
 
 
+  <scenario label="graph:make-from-vertices">
+    <scenario label="with no vertices">
+      <call function="graph:make-from-vertices">
+        <param name="vertices"
+               select="()" />
+      </call>
+
+      <expect label="produces an empty graph">
+        <preproc:sym-deps />
+      </expect>
+    </scenario>
+
+
+    <scenario label="with vertices">
+      <call function="graph:make-from-vertices">
+        <param name="vertices"
+               select="$foo:graph-vtwo/preproc:sym-dep" />
+      </call>
+
+      <expect label="produces a graph containing those vertices"
+              select="$foo:graph-vtwo" />
+    </scenario>
+
+
+    <scenario label="with duplicate vertices">
+      <call function="graph:make-from-vertices">
+        <param name="vertices"
+               select="$foo:graph-with-dupes/preproc:sym-dep" />
+      </call>
+
+      <expect label="removes duplicate vertices and edges"
+              select="$foo:graph-deduped" />
+    </scenario>
+  </scenario>
+
+
   <scenario label="graph:reverse on a disconnected DAG">
     <call function="graph:reverse">
       <param name="graph"