in immutable data you dont set things twice... see haskell and scala vals (and clojure of cource)...
for example.. for a data structure.. like a tree, when you perform write operation to the tree, in fact you are adding elements outside of the immutable tree.. after you done.. the tree and the branch are recombined in a new tree.. so like this you could perform concurrent reads and writes very safelly..
in tradicional model, you must lock a value cause it could be reseted any time.. so.. you end up with a very heat zone for threads..since they act sequentially there anyway..
with imuttable data, you dont set things more than once.. its a whole new way of programming.. you may end up using a little bit more memory.. but parallelizing is natural and painless..
