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
  (cot | [.PI/4,3/4*PI.]) * (arccot | [.-1,1.]) = id [.-1,1.] by Th22,Th26,
