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

  rr for set,

  rseq for Real_Sequence;

theorem Th67:
  arccosec1 * (cosec | [.-PI/2,0.[) = id [.-PI/2,0.[ by Lm27,Th27,FUNCT_1:39;
