he paper propose scheme for search in encrypted data using the tree of bloom filters as data structure to represent the index of the files then encrypted the leaves of the tree which consist of bloom filter then encrypt the inner nodes which is Xor of the bloom filter leaves using the following that i can not understand : for each internal node u in the index tree,  hash table Y, and for every i=1,2,...,n, set Yu[ f (sk, Wi)]=g(f ( sk, Wi), Du[i]).  where f is pseudorandom function, defined as :{o,1} ^* multiply sk--> {0,1}^k, sk is secret key.

and g(.,.) is pseudorandom function , defined as :{0,1}^k multiply {0,1}--> {0,1}^l . and Du is the inner node . please i want anyone explain me that with simple example.