| BooleanRule | 
 The rule for boolean is always an EQUAL rule. 
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| ClosedChoice | 
 The rule covers the XMP types "closed choice". 
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| DateRule | 
 This rule applies for the type "Date". 
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| IntegerRule | 
 This rule models a comparison of an Integer property with a constant argument. 
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| LengthRule | 
 This rule applies for the type "Text" (string length) and 
 all arrays (number of array items). 
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| LogicalRule | 
 The logical rule describes a conjunction (AND), disjunction (OR) or negation (NOT)
 of the child rules. 
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| OpenChoice | 
 The rule covers the XMP types "open choice". 
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| PatternRule | 
 This rule represents a regular expression that has to match with the property value. 
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| RealRule | 
 This rule is a comparison of a Real property with a constant argument. 
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| Vocable | 
 A vocable is one item that is part of the OpenChoice and ClosedChoice rules. 
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