code: public static <A> Lens.Simple<Set<A>, Boolean> contains(A a) {
          return contains(HashSet::new, a);
      }
docs: A lens that focuses on whether a {@link Set} contains some value <code>a</code>. Like {@link #contains(Function,
  Object)} but with an implicit copy function that produces <code>{@link HashSet}s</code>.
  
  @param a   the value in question
  @param <A> the value type
  @return a lens that focuses on a value's inclusion in a given {@link Set}