input: Write a function to compare two numbers and print which one is greater.
  assert compare_numbers(10, 5) == 'a is greater than b'
  assert compare_numbers(5, 10) == 'b is greater than a'
  assert compare_numbers(7, 7) == 'a is equal to b'
output: def compare_numbers(a, b):
      if a > b:
          return 'a is greater than b'
      elif a < b:
          return 'b is greater than a'
      else:
          return 'a is equal to b'