<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def fix_indentation(self, index=None):

 """Replace tab characters by spaces"""

        if index is None:
            index = self.get_stack_index()
        finfo = self.data[index]
        finfo.editor.fix_indentation()