code: public Boolean getBoolean (String key, Supplier<Boolean> notFound) {
          return getParsed (key, Boolean::new, notFound);
      }
docs: Retrieve a mapped element and return it as a Boolean.
  
  @param key A string value used to index the element.
  @param notFound A function to create a new Boolean if the requested key was not found
  @return The element as a Boolean, or notFound if the element is not found.