class HexaPDF::Type::Annotations::MarkupAnnotation

See: PDF2.0 s12.5.6.2, HexaPDF::Type::Annotation
annotations are actually markup annotations.
Markup annotations are used to “mark up” a PDF document, most of the available PDF

def perform_validation #:nodoc:

:nodoc: 
def perform_validation #:nodoc:
  super
  if key?(:RT) && !key?(:IRT)
    yield("/IRT required if /RT field is set")
  end
end