code: public static <K, V> Lens.Simple<Map<K, V>, V> valueAt(K k, V defaultValue) {
          return adapt(unLiftB(unLiftA(valueAt(k), defaultValue)));
      }
docs: A lens that focuses on a value at a key in a map, falling back to <code>defaultV</code> if the value is missing.
  <p>
  Note that this lens is NOT lawful, since "putting back what you got changes nothing" fails for any value
  <code>B</code> where <code>S</code> is the empty map
  
  @param k            the key to focus on
  @param defaultValue the default value to use in case of a missing value at key
  @param <K>          the key type
  @param <V>          the value type
  @return a lens that focuses on the value at the key