Definition inE := (mem_seq1, in_cons, inE).
Prenex Implicits mem_seq1 uniq undup index.
Arguments eqseq {T} !_ !_.
Arguments eqseqP {T x y}.
Arguments hasP {T a s}.
Arguments hasPn {T a s}.
Arguments allP {T a s}.
Arguments allPn {T a s}.
Arguments nseqP {T n x y}.
Arguments count_memPn {T x s}.
Section NthTheory.

Lemma nthP (T : eqType) (s : seq T) x x0 :
reflect (exists2 i, i < size s & nth x0 s i = x) (x \in s).