from collections import Counter

def minimum_cover(source, target):
    if not target:
        return ''
    need = Counter(target)
    missing = len(target)
    left = best_start = 0
    best_length = len(source) + 1
    for right, char in enumerate(source):
        if need[char] > 0:
            missing -= 1
        need[char] -= 1
        while missing == 0:
            length = right - left + 1
            if length < best_length:
                best_start, best_length = left, length
            old = source[left]
            need[old] += 1
            if need[old] > 0:
                missing += 1
            left += 1
    if best_length > len(source):
        return ''
    return source[best_start:best_start + best_length]