def record_uri
      lambda do |parsed|
        raise "Tried to access subject URI for #{parsed.record}, " \
        "but it is not saved." unless parsed.record.exists?
        parsed.record.rdf_subject
      end
    end
def record_uri
      lambda do |parsed|
        raise "Tried to access subject URI for #{parsed.record}, " \
        "but it is not saved." unless parsed.record.exists?
        parsed.record.rdf_subject
      end
    end
Gives access to a delayed call for the `#rdf_subject` of an
 OriginalRecord, to be executed at the time the mapping is processed.

 @return [Proc] a proc that, when called, returns the #rdf_subject of the
   OriginalRecord associated with the parsed record passed as its argument.