code: private Result listSessions() {
          StringBuilder sb = new StringBuilder("<!DOCTYPE html>\n" +
                  "<html lang=\"en\">\n" +
                  "<head>\n" +
                  "        <meta charset=\"utf-8\">\n" +
                  "        <title>Training sessions - DL4J Training UI</title>\n" +
                  "    </head>\n" +
                  "\n" +
                  "    <body>\n" +
                  "        <h1>DL4J Training UI</h1>\n" +
                  "        <p>UI server is in multi-session mode." +
                  " To visualize a training session, please select one from the following list.</p>\n" +
                  "        <h2>List of attached training sessions</h2>\n");
          if (!knownSessionIDs.isEmpty()) {
              sb.append("        <ul>");
              for (String sessionId : knownSessionIDs.keySet()) {
                  sb.append("            <li><a href=\"train/" + sessionId + "\">" + sessionId + "</a></li>\n");
              }
              sb.append("        </ul>");
          } else {
              sb.append("No training session attached.");
          }
  
          sb.append("    </body>\n" +
                  "</html>\n");
          return ok(sb.toString()).as("text/html; charset=utf-8");
      }
docs: List training sessions
  @return HTML list of training sessions