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

  rr for set,

  rseq for Real_Sequence;

theorem
  (sec | [.3/4*PI,PI.]) qua Function * (arcsec2 | [.-sqrt 2,-1.]) = id
  [.-sqrt 2,-1.] by Th42,Th50,FUNCT_1:39;
