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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def depth(self):
 """Increase the level of indentation by one."""

        if self.indentation is None:
            yield
        else:
            previous = self.previous_indent
            self.previous_indent = self.indent
            self.indent += self.indentation
            yield
            self.indent = self.previous_indent
            self.previous_indent = previous