@Override
    public String toString() {
        String previousCall = "";
        if (callStack != null) {
            Call call = callStack.getLastPopped();
            if (call != null) {
                previousCall = "Last call: " + call;
            }
        }
        if (documentOperations.size() == 1) {
            return "Processing of " + documentOperations.get(0) + ". " + previousCall;
        } else {
            String listString = documentOperations.toString();
            if (listString.length() > 100) {
                listString = listString.substring(0, 99);
                listString += "...]";
            }

            return "Processing of " + listString + ". " + previousCall;
        }
    }
@return time left or {@link #NO_TIMEOUT} if processing has no timeout
docproc/src/main/java/com/yahoo/docproc/Processing.java