'Find the source line references by C{self.frame} and yield, in source
line order, it and the previous and following lines.
@return: A generator which yields two-tuples.  Each tuple gives a source
line number and the contents of that source line.'

def _getSourceLines(self):

    filename = self.frame[1]
    lineNumber = self.frame[2]
    for snipLineNumber in range((lineNumber - 1), (lineNumber + 2)):
        (yield (snipLineNumber, linecache.getline(filename, snipLineNumber).rstrip()))
