input: Write a function to calculate the harmonic mean of two numbers.
  assert harmonic_mean(7, 9) == 7.875
  assert harmonic_mean(3, 4) == 3.4285714285714284
  assert harmonic_mean(10, 10) == 10.0
output: def harmonic_mean(a, b):
      return 2 * (a * b) / (a + b)