#31 Same attribute as inst and no inst param

[Simone Masoni]

Currently function definition don't allow to define the
same attribute as inst and no inst param. This feature
could be useful in some applications.

One trick is to define a local function that retrieves
the cur_ value of a inst param.

[G.A. Cignoni]

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The general solution is to have Id and refId:
<GS:Param id="formal_old" refId="actual" inst="false">
<GS:Param id="formal_new" refId="actual" inst="true">