question about sublists



Hi,
  Given a list, say xx: [2,1,3,3,5], how would I use 'sublist' to
  extract the indices where '3' occurs?

   I know I can write a routine checking whether each element xx[i] = 3
   or not and print out the indices 'i'.

   What I want to know is how to implement a predicate in 'sublist'
   which will work.

TIA,
  -sen

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