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Attributes are in key-value form, and if there's no value provided for an attribute, a NotAvailableValueError will be raised.
 
               # File rss-0.2.9/lib/rss/rss.rb, line 149
def initialize(tag, value, attribute=nil)
  @tag, @value, @attribute = tag, value, attribute
  message = "value <#{value}> of "
  message << "attribute <#{attribute}> of " if attribute
  message << "tag <#{tag}> is not available."
  super(message)
end