fact: Fixpoint vb_strip inner outer := if outer is out0 :: out then if inner is inn0 :: inn then (inn0 <= out0 <= inn0.+1) && (vb_strip inn out) else (out0 == 1) && (vb_strip [::] out) else inner == [::].
imports: HB(structures) mathcomp(all_ssreflect) tools sorted partition