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 Th31:
  (arctan qua Function) * (tan | ].-PI/2,PI/2.[) = id ].-PI/2,PI/2
  .[ by Lm5,Th11,FUNCT_1:39;
