Magritte Validations

Anyone know of any good resources for working with the validation system of Magritte. Specifically I'm interested in what it takes to have fields that depend on the value of each other. Say an end date that must be before a start date, both of which have just been edited. I was thinking something like this on the instance side... description ^self class description collect: [:each | each accessor selector = #eventEndDate ifTrue: [each addCondition: [:value | value > self eventStartDate]] ifFalse: [each]] But that doesn't work because I can't rely on eventStartDate having a value yet, it may not have been set by its description, or it may be invalid for some other reason. Are the any public sample apps that use Magritte heavily one could look at? Ramon Leon http://onsmalltalk.com