Make sure you completely understand why these functions behave the way they do
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void version_one(const int n)
{ print("start ");
  print(n);
  new_line();

  if (n > 0)
    version_one(n-1);

  print("end");
  print(n);
  new_line(); }

version_one(4) prints 
  start 4 start 3 start 2 start 1 start 0 end 0 end 1 end 2 end 3 end 4

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void version_two(const int n)
{ print("start ");
  print(n);
  new_line();

  if (n > 0)
    version_two(n-1); }

version_two(4) is trhe more familiar form, it prints 
  start 4 start 3 start 2 start 1 start 0

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void version_three(const int n)
{
  if (n > 0)
    version_three(n-1);

  print("end");
  print(n);
  new_line(); }

version_three(4) prints 
  end 0 end 1 end 2 end 3 end 4

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void version_four(const int n)
{ const int quo = n / 10;
  const int rem = n % 10;
  if (quo > 0)
    version_four(quo);
  print(" ");
  print(rem); }

version_four(75396) prints 7 5 3 9 6, all the digits one-by-one.

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void version_five(const int n)
{ const int quo = n / 2;
  const int rem = n % 2;
  if (quo > 0)
    version_five(quo);
  print(" ");
  print(rem); }

version_five just replaces the tens with twos, and now it prints numbers in binary
version_five(22) prints 1 0 1 1 0

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