[Haskell-cafe] help with a question

[haonan21]

I'm very new to haskell hugs and would appreciate it if someone could help me
out here. I've been giving 2 questions. 

1.) A and B are two sets of integers. Implement a function to obtain the
integers that belong to both sets.
Test your function fully.

2.) Define and test a function f, which, if A is a set of {x, y, z} then
f(A) = {{},{x}, {y}, {z}, {x, y}, {x,z}, {y,z}, {x, y, z}}

Manage to get the first one.
interset::[Int]->[Int]->[Int]

interset x [] = []

interset [] y = []

interset x@(xs:xt) y@(ys:yt) =
 if xs == ys
 then as:(interset at yt)
 else interset at y

Totally have no clue for the 2nd question. could someone help me out ? 

Many thanks!
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