src/Pure/System/session.scala

       {
         for {
           digest <- command.blobs_digests
           if !global_state().defined_blob(digest)
         } {
-          doc_blobs.retrieve(digest) match {
+          doc_blobs.get(digest) match {
             case Some(blob) =>
               global_state >> (_.define_blob(digest))
               prover.get.define_blob(blob)
             case None =>
               System.err.println("Missing blob for SHA1 digest " + digest)