Add spanning tree example

diff --git a/cmd/gvpr/lib/span.g b/cmd/gvpr/lib/span.g
new file mode 100644 (file)
index 0000000..348a6fc
--- /dev/null
+++ b/cmd/gvpr/lib/span.g
@@ -0,0 +1,3 @@
+/* derive simple spanning tree */
+BEG_G { $tvtype = TV_dfs}
+E [!isSubnode($T,$.head) || !isSubnode($T,$.tail)]