Implement a function that returns all unique permutations of a given string, ensuring that the output is sorted lexicographically.

``` def sorted_unique_permutations(s): results = [] def backtrack(path, used): if len(path) == len(s): results.append(''.join(path)) return for i in range(len(s)): if used[i]: continue if i > 0 and s[i] == s[i - 1] and not used[i - 1]: continue used[i] = True path.append(s[i]) backtrack(path, used) path.pop() used[i] = False s = sorted(s) backtrack([], [False] * len(s)) return results # Example usages: print(sorted_unique_permutations('zyx')) # ['xyz', 'xzy', 'yxz', 'yzx', 'zxy', 'zyx'] print(sorted_unique_permutations('bca')) # ['abc', 'acb', 'bac', 'bca', 'cab', 'cba'] ```