reserve x,x0, r,r1,r2 for Real,
      th for Real,

  rr for set,

  rseq for Real_Sequence;

theorem Th26:
  rng arcsec2 = ].PI/2,PI.]
proof
  dom (sec|].PI/2,PI.]) = ].PI/2,PI.] by Th2,RELAT_1:62;
  hence thesis by FUNCT_1:33;
end;
