src/Pure/General/sha1.scala

     override def hashCode: Int = rep.hashCode
     override def toString: String = Library.terminate_lines(rep)
 
     def is_empty: Boolean = rep.isEmpty
 
+    def :::(other: Shasum): Shasum = new Shasum(other.rep ::: rep)
+
     def digest: Digest = {
       rep match {
         case List(s)
         if s.length == digest_length && s.forall(Symbol.is_ascii_hex) => fake_digest(s)
         case _ => SHA1.digest(toString)