Swap the values at indices i & j.

		.. versionadded:: 1.1

def swap(self, i, j):
		"""Swap the values at indices i & j.

		.. versionadded:: 1.1
		"""
		i = self._fix_neg_index(i)
		j = self._fix_neg_index(j)
		self._list[i], self._list[j] = self._list[j], self._list[i]
		self._dict[self._list[i]] = i
		self._dict[self._list[j]] = j