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

  rr for set,

  rseq for Real_Sequence;

theorem Th68:
  arccosec2 * (cosec | ].0,PI/2.]) = id ].0,PI/2.] by Lm28,Th28,FUNCT_1:39;
