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A set of ids representing the base of a link.
For a partial mapping where a number of 'to' elements are known to link to a number of 'from' elements it can be useful to aggregate these into a single attribute value. The primary use is to assert that n links exist between a set of n 'to' elements and n 'from' elements but that the precise links are unknown. The semantics of the reference are the same as for 'to' and all the elements must be of the same type (which can be specified with 'toType' either on the link or the containing map). No order information is implied. In general there will be the same number of idRefs in the 'fromSet' and all implicit links will share the same attributes (e.g. 'role'). In many cases the sets will be later split into discrete links thorugh further calculation or experiment (e.g. peak assignment). Sets should never be used as a lazy or concise alternative where the all the links are explicitly known.