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
  arctan qua Function * (tan | ].-PI/2,PI/2.[) = id ].-PI/2,PI/2.[ by Lm5,Th11,
