reserve s for non empty typealg,
  T,X,Y,T9,X9,Y9 for FinSequence of s,
  x,y,z,y9,z9 for type of s;
reserve Tr for PreProof of s;
reserve p for Proof of s,
  v for Element of dom p;

theorem
  (p.v)`2 = 0 or ... or (p.v)`2 = 7
proof
  v is correct by Def12;
  hence thesis by Def4;
end;
