reserve x,x0, r, s, h for Real,

  n for Element of NAT,
  rr, y for set,
  Z for open Subset of REAL,

  f, f1, f2 for PartFunc of REAL,REAL;

theorem
  -1 <= r & r <= 1 & arccot r = PI/4 implies r = 1 by Th18,Th52;
