code: public static PathSpecification byKey(final String key) {
  
          checkNotNull(key);
  
          return new PathSpecification() {
  
              @Override
              public OptionalPath path(Bindings bindings) {
  
                  checkNotNull(bindings);
  
                  Binding<Config> config = bindings.get(Config.class);
  
                  if (config.isPresent()) {
  
                      try {
  
                          return new SomePath(
                              Paths.get(config.get().getString(key))
                          );
  
                      } catch (ConfigException.Missing ignored) { }
                  }
  
                  return NO_PATH;
              }
  
              @Override
              public String name() {
                  return String.format("by key: %s", key);
              }
          };
      }
docs: A {@link PathSpecification} that uses a {@link Config} to
  {@link Config#getString(String) get} a path.
  
  <p>The {@link PathSpecification#name() name} of the returned
  path specification is "by key: _", where "_" is substituted
  by the {@code key} parameter.</p>
  
  @param key
  The <i>path</i> (in the {@link Config} object) at which to look
  for a string representing a <i>path</i> (on the filesystem).