reserve a,b,c,k,m,n for Nat;
reserve i,j for Integer;
reserve p for Prime;

theorem Th3:
  n < 5 implies n = 0 or n = 1 or n = 2 or n = 3 or n = 4
  proof
    assume n < 5;
    then n < 4+1;
    then n <= 4 by NAT_1:13;
    then n = 0 or ... or n = 4;
    hence thesis;
  end;
