Goal:
  
  G1 : t
  G2 : t
  H : invariants G1
  H0 : invariants G2
  l : labelling
  HGl : correct_labelling G1 l
  embed : full_subgraph G1 G2
  acG2 : acyclic_no_loop G2
  Gl := relabel_on G1 G2 l : t
  l' := to_label ∘ lsp Gl (s Gl) : V.t -> nat
  x : VSet.elt
  xin1 : VSet.In x (V G1)
  p : PathOf G1 (s G1) x
  lsp_eq : lsp (relabel_on G1 G2 l) (s G1) x = Some (diff l (s G1) x)
  ============================
   (l' x = l x)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrcase@0> $1 $2
DEBUG LTAC Evaluated term: (source_pathOf G1 x xin1)
DEBUG LTAC Evaluated term: (lsp_Gl_between_G1 G1 G2 l 
                              (edges_sub G1 G2 embed) HGl
                              (lsp_dominate G1 G2 embed) 
                              (s G1) x (vertices_sub G1 G2 embed) p
                              (source_vertex G1) xin1)