combinations_problem

gistfile1.txt

You have a set A of n integers.

For every i ∈ A, 
i may or may not also belong to a replacement set B, which may be the empty set,
where any k ∈ B can replace i in the original set.

Each replacement set B can be of any size.

Is there a way (especially an efficient way) to generate
all combinations of size n (call it A') where:

all unique combinations A' where order is not important.  
Each i ∈ A' should also not be repeated within a given A'

Note that A' could be entirely composed of integers from replacements sets, no
original member of A need be preserved.