reserve r,r1,r2, s,x for Real,
  i for Integer;

theorem Th60:
  cos | [.PI+2*PI*i,2*PI+2*PI*i.] is one-to-one
proof
  set Q1 = [.PI+2*PI*i,2*PI+2*PI*i.];
  cos|Q1 is increasing by Th56;
  hence thesis by FCONT_3:8;
end;
