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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 is_valid_varname(self, name, item):
 """ Valid variable name, checked against global context. """

        check_valid_varname(name, self._custom_units, self._structs, self._constants, item)
        if name in self._globals:
            raise VariableDeclarationException(
                'Invalid name "%s", previously defined as global.' % name, item
            )
        return True