Return the first available quantity in the input arguments.
        Return `None` if none of them is available.

def first_available(self, *quantities):
        """
        Return the first available quantity in the input arguments.
        Return `None` if none of them is available.
        """
        for i, q in enumerate(quantities):
            if self.has_quantity(q):
                if i:
                    warnings.warn('{} not available; using {} instead'.format(quantities[0], q))
                return q