(** The elements of a multi-unit separation algebra can be partitioned
    into equivalance classes, where two elements are in the class iff
    they have the same unit.
  *)

  Definition comparable {A} `{Sep_alg A}  (a b:A)
    := core a = core b.

Lemma Sep_alg_ext {T} {J} {PA: @Perm_alg _ J}: forall (sa1 sa2: @Sep_alg T J), sa1=sa2.