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The context for the 'from' links in a map.
A reference to the unique 'id' attribute of an element defining the context for links in a map. This may be required when id attributes may not be unique within a document. The id should either reference an element uniquely or should be taken as the first ancestor (of the map) with such an id.
This is fairly horrid but may be required when documents are assembled without establishing unique ids (e.g. concatenation of files). As an example a map referencing linked atoms in two molecules might use the containing 'reaction' element as its uniquifying context.