function TableOfContents () {
    this.getSections = function() {
        /*
        - return an array of section objects
        - in case there there is no table of content, return an empty array
        */
    }

    this.scrollToSection = function(index: number) {
        /*
        - scroll to a section with index in array returned by this.getSections
        - perform change to the page so that the section become available for reading, such as expanding the collapsed section
        */
    }

    // OPTIONAL
    this.visibleSectionIndexes = function() {
        /*
        - return an array of visible section's indexes
        - in case there there is no table of content, return an empty array
        */
    }

    // OPTIONAL
    this.startVisibleSectionCallBack = function () {
        /*
        - start page visible section call back
        - using this call back, the reader app could know which section is currently visible to the user and highlighting the location in table of content
        - implementation to be discussed. It might not be a good idea to modify window.onscroll directly
        */
        var handleScroll = function() {
            var visible = tableOfContents.visibleSectionIndexes();
            if (visible.length > 0) {
                window.location = 'pagescroll:scrollEnded?start=' + visible[0] + '&length=' + visible.length;
            }
        }
        window.onscroll = handleScroll;
        handleScroll();
    }

    // OPTIONAL
    this.stopVisibleSectionCallBack = function () {
        /*
        - stop page visible section call back
        */
        window.onscroll = undefined;
    }

}

function Section(title: string, level: number) {}