theorem Th26:
  P1 is being_S-P_arc implies P1 <> {}
proof
  assume P1 is being_S-P_arc;
  then consider f such that
A1: f is being_S-Seq and
A2: P1 = L~f;
  len f >= 2 by A1;
  hence thesis by A2,Th23;
end;
