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
  (tan | [.-PI/4,PI/4.]) * (arctan | [.-1,1.]) = id [.-1,1.] by Th21,Th25,
