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

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