<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _findExpressionStart(self, block):
 """Find start of not finished expression Raise UserWarning, if not found """


        # raise expession on next level, if not found
        expEndBlock, expEndColumn = self._findExpressionEnd(block)

        text = expEndBlock.text()[:expEndColumn + 1]
        if text.endswith(')'):
            try:
                return self.findBracketBackward(expEndBlock, expEndColumn, '(')
            except ValueError:
                raise UserWarning()
        else:
            return expEndBlock, len(text) - len(self._lastWord(text))