#line 218 "/home/travis/build/felix-lang/felix/src/packages/algebra.fdoc"
  // roughly, a finite Set
  class Container [c,v]
  {
    inherit Set[c,v];
    virtual fun len: c -> size;
    fun \(\Vert\) (x:c) => len x;
    virtual fun empty(x: c): bool => len x == size(0);
  }