Add more machinery to use embeddings as "proofs of child-ship"

lean/Spa/Language/Program.lean

@@ -31,6 +31,12 @@ def cfg : Graph := Graph.wrap p.rootStmt.cfg
 /-- A state in the control flow `Spa.Graph` of this program. -/
 abbrev State : Type := p.cfg.Index
 
+/-- The root statement's CFG sits inside the program's CFG (that graph `wrap`ped
+    in an entry and an exit node). -/
+def rootEmbed : GGraph.Embed p.rootStmt.cfg p.cfg :=
+  (GGraph.Embed.sequenceLeft p.rootStmt.cfg (Graph.singleton none)).trans
+    (GGraph.Embed.sequenceRight (Graph.singleton none) _)
+
 /-- Variables mentioned or defined in this program. -/
 def vars : List String := p.rootStmt.vars.sort (· ≤ ·)