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Fixtype of pairs consisting of an optional value and a dynamic environment.
This is a product type introduced by fty::defprod.
In general, the execution of a statement may return a value (or not), and may affect the dynamic environment, by introducing new variables and constants and by modifying the content of existing variables. This fixtype captures these possible outcomes.
This fixtype is a dynamic counterpart of types+senv,
which is used in type checking.
In types+senv,
the set of optional types describes many possible optional return values,
which in this