diff --git a/src/graph.xsl b/src/graph.xsl
index 692f577b..387d1830 100644
--- a/src/graph.xsl
+++ b/src/graph.xsl
@@ -144,4 +144,50 @@
   </preproc:sym-deps>
 </function>
 
+
+<!--
+  Merge sequence of graphs @var{$graphs} into a single graph by taking
+    the union of all vertices and edges.
+  Directionality will be preserved.
+
+  Edge attributes (@code{preproc:sym-ref/@@*)} will be set to the
+    union of all attributes on all edges of the same @code{@@name}.
+  @emph{If edge attributes do not share the same value,
+    the behavior is undefined.}
+
+  For example:
+
+  @float Figure, fig:union-graph
+  @verbatim
+  G₁:  A->B->C
+  G₂:  C->A
+  G₃:  B->C->D
+
+  G∪:  A->B->C->D
+       ^____/
+  @end verbatim
+  @caption{(G₁ ∪ G₂ ∪ G₃)}
+  @end float
+-->
+<function name="graph:union" as="element( preproc:sym-deps )*">
+  <param name="graphs" as="element( preproc:sym-deps )*" />
+
+  <preproc:sym-deps>
+    <for-each-group select="$graphs/preproc:sym-dep"
+                    group-by="@name">
+      <preproc:sym-dep name="{@name}">
+        <for-each-group select="current-group()/preproc:sym-ref"
+                        group-by="@name">
+          <preproc:sym-ref>
+            <sequence select="current-group()/@*" />
+
+            <!-- keep our name (overrides the above) -->
+            <attribute name="name" select="@name" />
+          </preproc:sym-ref>
+        </for-each-group>
+      </preproc:sym-dep>
+    </for-each-group>
+  </preproc:sym-deps>
+</function>
+
 </stylesheet>
diff --git a/test/graph-test.xsl b/test/graph-test.xsl
index fea46fa9..63cf6186 100644
--- a/test/graph-test.xsl
+++ b/test/graph-test.xsl
@@ -99,6 +99,76 @@
 </variable>
 
 
+<variable name="foo:graph-empty" as="element( preproc:sym-deps )">
+  <preproc:sym-deps />
+</variable>
+
+
+<variable name="foo:graph-vtwo" 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" attr2="bar" />
+    </preproc:sym-dep>
+
+    <!-- test empty for merge -->
+    <preproc:sym-dep name="b" />
+  </preproc:sym-deps>
+</variable>
+
+
+<variable name="foo:graph-vthree" as="element( preproc:sym-deps )">
+  <preproc:sym-deps>
+    <preproc:sym-dep name="a">
+      <preproc:sym-ref name="b" attr1="foo" />
+      <preproc:sym-ref name="c" />
+    </preproc:sym-dep>
+
+    <preproc:sym-dep name="b">
+      <preproc:sym-ref name="c" attr3="baz" />
+    </preproc:sym-dep>
+
+    <preproc:sym-dep name="c">
+      <preproc:sym-ref name="a" />
+    </preproc:sym-dep>
+
+    <!-- disconnected -->
+    <preproc:sym-dep name="d">
+      <preproc:sym-ref name="e" />
+    </preproc:sym-dep>
+
+    <preproc:sym-dep name="e" />
+  </preproc:sym-deps>
+</variable>
+
+
+<!-- result of merging the above two -->
+<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-dep>
+
+    <preproc:sym-dep name="b">
+      <preproc:sym-ref name="c" attr3="baz" />
+    </preproc:sym-dep>
+
+    <preproc:sym-dep name="c">
+      <preproc:sym-ref name="a" />
+    </preproc:sym-dep>
+
+    <!-- disconnected -->
+    <preproc:sym-dep name="d">
+      <preproc:sym-ref name="e" />
+    </preproc:sym-dep>
+
+    <preproc:sym-dep name="e" />
+  </preproc:sym-deps>
+</variable>
+
+
 <function name="foo:lookup">
   <param name="yield"  as="element()" />
   <param name="symbol" as="element( preproc:sym )" />
diff --git a/test/graph.xspec b/test/graph.xspec
index 14a04efd..c5e32163 100644
--- a/test/graph.xspec
+++ b/test/graph.xspec
@@ -171,4 +171,50 @@
                       $edge/@cattr = 'cvalue'
                         and $edge/@cattr2 = 'cvalue2'" />
   </scenario>
+
+
+  <scenario label="graph:union">
+    <scenario label="given nothing">
+      <call function="graph:union">
+        <param name="graphs"
+               select="()" />
+      </call>
+
+      <expect label="produces an empty graph"
+              select="$foo:graph-empty" />
+    </scenario>
+
+
+    <scenario label="given two empty graphs">
+      <call function="graph:union">
+        <param name="graphs"
+               select="( $foo:graph-empty, $foo:graph-empty )" />
+      </call>
+
+      <expect label="produces an empty graph"
+              select="$foo:graph-empty" />
+    </scenario>
+
+
+    <scenario label="given graph with empty">
+      <call function="graph:union">
+        <param name="graphs"
+               select="( $foo:graph-empty, $foo:graph-vtwo )" />
+      </call>
+
+      <expect label="produces the non-empty graph"
+              select="$foo:graph-vtwo" />
+    </scenario>
+
+
+    <scenario label="given two different graphs">
+      <call function="graph:union">
+        <param name="graphs"
+               select="( $foo:graph-vtwo, $foo:graph-vthree )" />
+      </call>
+
+      <expect label="merges vertices and edges of unique vertices"
+              select="$foo:graph-vtwo-vthree" />
+    </scenario>
+  </scenario>
 </description>