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

  rr for set,

  rseq for Real_Sequence;

theorem
  (cosec | [.PI/4,PI/2.]) qua Function * (arccosec2 | [.1,sqrt 2.]) = id
  [.1,sqrt 2.] by Th44,Th52,FUNCT_1:39;
