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
  arccot qua Function * (cot | ].0,PI.[) = id ].0,PI.[ by Lm6,Th12,FUNCT_1:39;
