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"An annotation constrained to appear only on certain program 
 elements, and only with certain values. 
 
 This interface should never be satisfied directly by any
 annotation type. Instead, either [[OptionalAnnotation]] or 
 [[SequencedAnnotation]] should be satisfied by the 
 annotation type.
 
 The type parameters encode information about the annotation
 type and its constraints:
 
  - [[Value]] represents the type of the annotation itself, 
  - [[ProgramElement]] represents a constraint on the  
    _reference expression type_ of the annotated program 
    element, for example, 
    [[ceylon.language.meta.declaration::ClassDeclaration]] 
    or [[ceylon.language.meta.declaration::Module]], where
    [[Annotated]] means there is no constraint, and
  - [[Type]] is a constraint on the _metamodel type_ of the 
    annotated program element, for example, 
    [[`Function`
     |ceylon.language.meta.model::Function]], 
    where `Anything` means there is no constraint, and that 
    the program element need not have a metamodel type."
see (`interface Annotation`,
     `interface OptionalAnnotation`,
     `interface SequencedAnnotation`)
shared interface ConstrainedAnnotation 
        of Value
        //Note: adding the following constraint would
        //      make ConstrainedAnnotation a GADT, which
        //      the language does not currently support 
        //of OptionalAnnotation | 
        //   SequencedAnnotation
        satisfies Annotation
        given Value satisfies Annotation
        given ProgramElement satisfies Annotated {

}