View source for AUI expressions and constraints

There is a need for constraints on individual interactors and across interactors. Some of this should be expressed as part of the domain model, but some is specific to the abstract UI.

''This is inspired by the capabilities in W3C's XForms specifications''

* Constraints on individual field
** Value in given range or matches a given a pattern
* Constraints across fields
** Field as summation of other field values
** Field value must be less than some other field
** Cardinality constraints on number of answers
** Value required if some other field has a given value
* Model Attributes
** Disabled, required, default, relevant, ...

We separately discussed requirements for security and access control, as well as constraints that are effected by environmental factors for context aware applications.