@Override
  public String toString() {
    final StringWriter writer = new StringWriter();
    final PrintWriter printer = new PrintWriter(writer);
    printer.append(getClass().getSimpleName());
    printer.append("{");
    printer.print(operator().name());
    DebugExpressionVisitor<PrintWriter> debug = new DebugExpressionVisitor<>(printer);
    arguments().forEach(a -> {
      a.accept(debug);
    });
    printer.append("}");
    return writer.toString();
  }
Get the operator symbol for this operation

@return operator
criteria/common/src/org/immutables/criteria/expression/Call.java