input: Write a python function to create a command processing loop with 'help', 'start', and 'stop' commands. 
  assert process_command(["help"]) == ["Available commands are: 'help', 'start', 'stop'"]
  assert process_command(["start", "stop"]) == ["", ""]
  assert process_command(["foo", "help"]) == ["Invalid command.", "Available commands are: 'help', 'start', 'stop'"]
output: def process_command(commands):
      results = []
      for command in commands:
          if command == "help":
              results.append("Available commands are: 'help', 'start', 'stop'")
          elif command == "start":
              results.append("")
          elif command == "stop":
              results.append("")
          else:
              results.append("Invalid command.")
      return results