<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def transform_current_line(self, transform_callback):
 """ Apply the given transformation function to the current line. :param transform_callback: callable that takes a string and return a new string. """

        document = self.document
        a = document.cursor_position + document.get_start_of_line_position()
        b = document.cursor_position + document.get_end_of_line_position()
        self.text = (
            document.text[:a] +
            transform_callback(document.text[a:b]) +
            document.text[b:])