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

  rr for set,

  rseq for Real_Sequence;

theorem Th25:
  rng arcsec1 = [.0,PI/2.[
proof
  dom (sec|[.0,PI/2.[) = [.0,PI/2.[ by Th1,RELAT_1:62;
  hence thesis by FUNCT_1:33;
end;
