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

  rr for set,

  rseq for Real_Sequence;

theorem Th73:
  arcsec1.1 = 0 & arcsec1.(sqrt 2) = PI/4
proof
A1: arcsec1.1 = arcsec1 1 .= 0 by Th31,Th69;
A2: PI/4 < PI/2 by Lm5,XXREAL_1:3;
  arcsec1.(sqrt 2) = arcsec1 (sqrt 2) .= PI/4 by A2,Th31,Th69;
  hence thesis by A1;
end;
