docs: def clean(self):
          """
          Checks for the identification and password.
  
          If the combination can't be found will raise an invalid sign in error.
  
          """
          identification = self.cleaned_data.get('identification')
          password = self.cleaned_data.get('password')
  
          if identification and password:
              self.user_cache = authenticate(identification=identification, 
                                  password=password)
              if self.user_cache is None:
                  raise forms.ValidationError(_(u"Please enter a correct "
                          "username or email address and password. "
                          "Note that both fields are case-sensitive."))
          return self.cleaned_data