Ensure collection can have an element added or removed
public String validateAddTo<PropertyName>(<property type> param)
public String validateRemoveFrom<PropertyName>(<property type> param)
Complements a get method for a collection property.
This validate method is used to check that the specified object can be added to or removed
from a collection. Using such a mechanism the programmer can guarantee that the collection
never becomes invalid. If a String object is returned then adding/removing the specified object
is not valid and the collection will not be changed. The string itself will then be made visible to
user to inform them why changing the collection with the object would be invalid. If the method
returns null then the object will be added to/removed from the collection, that is, its
addTo/removeFrom method will be called with the same parameter.