The @keys attribute
 
 
 A @keys attribute consists of one or more space-separated keys. Map
  authors define keys using a <topicref> or <topicref>
  specialization that contains the @keys attribute. Each key definition introduces
  an identifier for a resource referenced from a map. Keys resolve to the resources given as the
   @href value on the key definition <topicref> element, to
  content contained within the key definition <topicref> element, or
  both.
The 
   @keys attribute uses the
                                following syntax:- The value of the @keysattribute is one or more space-separated key names.
- Key names consist of characters that are legal in a URI. The case of key names is significant.
- The following characters are prohibited in key names: "{", "}", "[", "]", "/", "#", "?", and whitespace characters.
A key cannot resolve to sub-topic elements, although a
     @keyref attribute can do so by combining a key
    with a sub-topic element id.
