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

  rr for set,

  rseq for Real_Sequence;

theorem Th28:
  rng arccosec2 = ].0,PI/2.]
proof
  dom (cosec|].0,PI/2.]) = ].0,PI/2.] by Th4,RELAT_1:62;
  hence thesis by FUNCT_1:33;
end;
