def split_def_command(command):
    """Split command in parts: variable name, specific command, definition.
    
    Return None if command is invalid.
    """
    # procedo per casi
    # il che significa che dopo un '@' non posso usare un ':'
    #   perché verrebbe intercettato per primo!
    valid_commands = [":", "@"]
    specific_command = "--invalid--"
    for comm_char in valid_commands:
        comm_pos = command.find(comm_char)
        if comm_pos > -1:
            specific_command = comm_char
            break
    if "--invalid--" == specific_command:
        return None
    
    return command.partition(specific_command)