code: Object exec(Element element, Object scope, String operand, Object... arguments) throws IOException, TemplateException
    {
      try {
        return doExec(element, scope, operand, arguments);
      }
      catch(TemplateException exception) {
        log.warn("Templates exception:\r\n" + //
            "\t- element: %s\r\n" + //
            "\t- operator: %s\r\n" + //
            "\t- operand: %s\r\n" + // " +
            "\t- scope: %s\r\n" + // " +
            "\t- cause: %s", element.trace(), getClass(), operand, scope, exception);
        throw exception;
      }
    }
docs: Execute operator. Execute operator logic into element context and returns a value; depending on specific operator
  implementation not all declared parameters may be used and returned type may vary, including void when return null. Operand
  string can denote a property path, an expression or a qualified class name, see every operator description.
  <p>
  A property path is used to access content values and can be absolute, prefixed with dot or relative to scope object. An
  expression is operator specific and should contain also some means to identify a content values. The point is,
  <em>scope</em>, <em>operand</em> tuple is used to somehow identify the content value(s) that will be injected by operator.
  
  @param element element on which operator is declared,
  @param scope scope object, used when operand is a property path,
  @param operand declared operand,
  @param arguments optional, operator type specific, argument(s).
  @return operator specific value or null.
  @throws IOException if underlying writer fails to write.
  @throws TemplateException if content value not found or of bad type.