code: @Override
      void completeCandidates(CommandContext ctx, String buffer, int cursor, List<String> candidates) {
          if (candidates.isEmpty()) {
              if (buffer.startsWith("\"") && buffer.length() >= 2) {
                  // Quotes are added back by super class.
                  buffer = buffer.substring(1);
              }
              if (buffer.length() == 2 && buffer.endsWith(":")) {
                  candidates.add(buffer + File.separator);
              }
          }
      }
docs: The only supported syntax at command execution is fully quoted, e.g.:
  "c:\Program Files\..." or not quoted at all. Completion supports only
  these 2 syntaxes.