rwa [skew_iff_cl_skew hX.symm.subset_ground hY.symm.subset_ground, hX.cl_eq_cl, hY.cl_eq_cl,
    ← skew_iff_cl_skew]
case refine_2
ι : Type u_1
α : Type u_2
M : Matroid α
B I J X X' Y Y' F : Set α
Xs Ys Js Is : ι → Set α
i j : ι
e f : α
hX : M.LoopEquiv X X'
hY : M.LoopEquiv Y Y'
h : M.Skew X' Y'
⊢ M.Skew X Y
no goals