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27 let rec sub_ex v1 v2 =
31 | s1::_, s2::_ when s1 < s2 -> false
32 | s1::_, s2::tl2 when s2 < s1 -> sub_ex v1 tl2
33 | s1::tl1, s2::tl2 -> sub_ex tl1 tl2