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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 sign_in(self, email=None, password=None):
 """Signs in a user, either with the specified email address and password, or a returning user."""

        from .pages.sign_in import SignIn
        sign_in = SignIn(self.selenium, self.timeout)
        sign_in.sign_in(email, password)