def print_source_line(msg, lineno, line, event_str=None):
<EOL>return msg('<STR_LIT>' % (event_str, lineno, line))<EOL>
Print out a source line of text , e.g. the second
    line in:
        (/tmp.py:2):  <module>
        L -- 2 import sys,os
        (trepan3k)

    We define this method
    specifically so it can be customized for such applications
    like ipython.