BEGIN  { 
  Trainf="train.arff"; Testf="test.arff"; 
  Bins=3; 
  Bin=1; 
  Seed=1;
} 
/^[ \t]*$/          { next }
/@relation/         { Seed ? srand(Seed) : srand(1)      }
/@relation/         { printf "">Trainf;  printf "">Testf }
/@relation/,/@data/ { print $0 >> Trainf;  print $0 >> Testf; next }
                    { Line[rand()] = $0; Lines++ }
END { 
  Start = Lines/Bins * (Bin - 1) ;
  Stop  = Lines/Bins * Bin;
  for(I in Line) {
    N++;
    What = (N>= Start && N < Stop) ? Testf : Trainf
    print Line[I]>>What; }
}