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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 to_set(self):
        '''Convert the source sequence to a set.

        Note: This method uses immediate execution.

        Raises:
            ValueError: If duplicate keys are in the projected source sequence.
            ValueError: If the Queryable is closed().
        '''
        if self.closed():
            raise ValueError("Attempt to call to_set() on a closed Queryable.")

        if isinstance(self._iterable, set):
            return self._iterable
        s = set()
        for item in self:
            if item in s:
                raise ValueError("Duplicate item value {0} in sequence "
                    "during to_set()".format(repr(item)))
            s.add(item)
        # Ideally we would close here
        #self.close()
        return s