Establishes a 'cascade' constraint property for validateable objects. If "cascade:true" is set on a nested object, the nested object's validate() method will be invoked and the results will be reported as part of the parent object's validation. Based on a blog post by Eric Kelm:  http://asoftwareguy.com/2013/07/01/grails-cascade-validation-for-pogos/ Used with permission.