have he' : (M.parallelExtend e f).Nonloop e := by
  rw [parallelExtend_nonloop_iff he]; left; assumption
α : Type u_1
inst✝ : DecidableEq α
M M' : Matroid α
e f✝ x : α
I C : Set α
he : M.Nonloop e
f : α
⊢ (M.parallelExtend e f).Parallel e f
α : Type u_1
inst✝ : DecidableEq α
M M' : Matroid α
e f✝ x : α
I C : Set α
he : M.Nonloop e
f : α
he' : (M.parallelExtend e f).Nonloop e
⊢ (M.parallelExtend e f).Parallel e f