theorem
  for s being State of S, l being Nat
  holds DataPart s = DataPart(s +* Start-At(l,S))
proof
  let s be State of S;
  let l be Nat;
  thus DataPart s =  DataPart s +* {}
      .=  DataPart s +* DataPart Start-At(l,S) by Th20
      .= DataPart(s +* Start-At(l,S))by FUNCT_4:71;
end;
