reserve a,b,r for non unit non zero Real;
reserve X for non empty set,
        x for Tuple of 4,X;
reserve V             for RealLinearSpace,
        A,B,C,P,Q,R,S for Element of V;

theorem Th20:
  for D being set
  for d being Element of D holds Seg 1 --> d = <* d *>
  proof
    let D be set;
    let d be Element of D;
    Seg 1 --> d = 1 |-> d
               .= <* d *> by FINSEQ_2:59;
    hence thesis;
  end;
