Theorem degree_L1_merge_summary: forall (m:fmap)(x y z:dart), let y0 := cA m zero y in let m1 := L m one x y in inv_hmap m1 -> exd m z -> ~expf m x y0 -> let dx:= MF.degree m x in let dy0:= MF.degree m y0 in MF.degree m1 z = if expf_dec m x z then dx + dy0 else if expf_dec m y0 z then dx + dy0 else MF.degree m z.