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 0 = 0 & arctan.0 = 0
proof
  arctan 0 = 0 by Th35,Th41;
  hence thesis;
end;
