Goal:
  
  G : t
  s1 : VSet.t
  s' : VSet.t
  x0 : V.t
  y0 : V.t
  z : V.t
  p0 : SPath s1 x0 y0
  s0 : VSet.t
  s2 : VSet.t
  x : V.t
  y : V.t
  z' : V.t
  da : DisjointAdd x s0 s2
  e : EdgeOf x y
  p : SPath s0 y z'
  hin : Disjoint s2 s'
  q : SPath s' z' z
  ============================
   (VSetProp.Add x (VSet.union s0 s') (VSet.union s2 s') /\
    ~ VSet.In x (VSet.union s0 s'))


DEBUG:Going to execute:
let tac2 H :=
            let ph := fresh "H" in
            let ph' := fresh "H" in
            destruct H as [H ph ph']
           in
           let tacT H := let ph := fresh "X" in
                         destruct H as [H ph] in
           let tacT2 H :=
            let ph := fresh "X" in
            let ph' := fresh "X" in
            destruct H as [H ph ph']
           in
           match goal with
           | H:exists y, _ y |- _ => tac H
           | H:{x : _ | ?P x} |- _ => tac H
           | H:∑ y, ?P y |- _ => tacT H
           | H:exists2 x : _, _ x & _ x |- _ => tac2 H
           | H:{x : _ | ?P x & _ x} |- _ => tac2 H
           | H:{x : _ & ?P x & _ x} |- _ => tacT2 H
           end