Python でネストされたリストの共通部分を効率的に見つけるにはどうすればよいですか?

b1 = [1,2,3,4,5,9,11,15]
b2 = [4,5,6,7,8]
print(set(b1) & set(b2))  # Output: {4, 5}

c1 = [1, 6, 7, 10, 13, 28, 32, 41, 58, 63]
c2 = [[13, 17, 18, 21, 32], [7, 11, 13, 14, 28], [1, 5, 6, 8, 15, 16]]

print(set(c1) & set(c2))  # Output: set([])

c3 = [[13, 32], [7, 13, 28], [1, 6]]

# Convert nested lists to sets 
set_c1 = set(c1) 
set_c2 = [set(sublist) for sublist in c2]

# Compute intersections
intersections = [set_c1.intersection(sublist) for sublist in set_c2]

# Reconstruct nested list structure 
result = [[item for item in intersection] for intersection in intersections] 

# Print the result
print(result)  # Output: [[13, 32], [7, 13, 28], [1, 6]]