get_recdef now returns None instead of raising ERROR.
This directorty contains the higher order quotients in Isabelle HOL
higher order quotients use partial equivalence relations/classes (PERs)
instead of euivalence relations/classes
We have two classes er<per
Quotients are a specialization of higher order quotients.
The use the same constructor quot with the subclass er.
An Example for the application of quotients are the fractions.
The example shows how to define an equivalence relation and how to use
the quotients.
For a more detailed description see [Slo97].