I have an abstract concept `Rational` for rational numbers. It has two concrete subconcepts, `Decimal` and `Fraction`, which represent two ways of entering rational numbers. I have defined editors for the concrete subconcepts. Now wherever editor for `Rational` is needed, I would like to always start with an editor for `Decimal`. (`Fraction` editor will be reachable by e.g. hitting '/' in the `Decimal` editor.) How would I instruct MPS to use `Decimal` as the default editor for `Rational`?