input: Write a function to perform basic arithmetic operations (addition, subtraction, multiplication, division) based on a command argument.
  assert arithmetic_operation('add', 4, 5) == 9
  assert arithmetic_operation('sub', 10, 3) == 7
  assert arithmetic_operation('mul', 6, 7) == 42
  assert arithmetic_operation('div', 8, 2) == 4.0
output: def arithmetic_operation(command, a, b):
      if command == 'add':
          return a + b
      elif command == 'sub':
          return a - b
      elif command == 'mul':
          return a * b
      elif command == 'div':
          return a / b