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{2 Overview}

The Sparecode module aims at removing the unused code.

It is composed of to parts : 
- one (in module {!module:Marks}) that computes some information
to say what has to be kept in the result. It uses the generic PDG 
marking facility {{:../pdg/PdgMarks.html}PdgMarks}+{{:../pdg/Marks.html}Marks},
- and a second one (module {!module:Transform})
that read thoses results to produce a new application.
This part mainly use {{:../html/Filter.html}Filter}
which provides a functor that filters an application to create another one.

To select the useful statements, we start from the [main] outputs
and the reachable annotations, and mark backward all the dependencies.
When reaching a function call, the called function statements are also marked
according to the needed outputs, but the inputs are not propagated immediately
because it would make every function call visible.
The information provided by the PDG marking system is kept to be used later.

So, after the first step, we iterate on the input marks to propagate,
and propagate them only for the visible calls, ie those which have at least
one visible output. This process is repeated as long as there are some
modification.

