code: public boolean is(@NotNull Key<?> key) {
          final Optional<?> v = get(key);
          return v.isPresent() && !v.get().equals(Boolean.FALSE);
      }
docs: Returns {@code true} if the given key exists and does not map to a boolean value of {@code false}.
  
  <p>So any other object, such as a string for example (even if empty) or an Integer
  (even with value 0) will return true.
  
  <p>Remember that this object contains no <code>null</code> values, see {@link #with}, thus
  after "adding" <code>null</code> this <code>'is'</code> check returns false.</p>