fact: Fixpoint head_at (h:Hydra)(o:occurrence) {struct h}: bool := match h, o with head, nil => true | node hs, o => head_at_l hs o | _, _ => false end with head_at_l (hs:Hydrae)(o:occurrence){struct hs} : bool := match hs, o with single h, 0::o' => head_at h o' | hcons h hs', 0::o' => head_at h o' | hcons h hs', S p::o' => head_at_l hs' (p::o') | _, _ => false end.
imports: Arith Omega List EPSILON0 MSE0