theorem Th78:
  n is even implies n mod 4 = 0 or n mod 4 = 2
  proof
    assume n is even;
    then consider m being Nat such that
A1: n = 2*m;
A2: (2*m) mod (2*2) = 2*(m mod 2) by INT_4:20;
    m mod 2 = 0 or m mod 2 = 1 by NAT_D:12;
    hence thesis by A1,A2;
  end;
