    var spans = Array.prototype.filter.call(
        sourcelines.children,
        function(x) { return x.tagName === 'SPAN' });

    // add a "followlines-select" class to change cursor type in CSS
    for (var i = 0; i < spans.length; i++) {
        spans[i].classList.add('followlines-select');
    }

    var lineSelectedCSSClass = 'followlines-selected';

    //** add CSS class on  element in `from`-`to` line range */
    function addSelectedCSSClass(from, to) {
        for (var i = from; i <= to; i++) {
            spans[i].classList.add(lineSelectedCSSClass);
        }
    }

    //** remove CSS class from previously selected lines */
    function removeSelectedCSSClass() {
        var elements = sourcelines.getElementsByClassName(
            lineSelectedCSSClass);
        while (elements.length) {
            elements[0].classList.remove(lineSelectedCSSClass);
        }
    }

    // ** return the  element parent of `element` */
    function findParentSpan(element) {
        var parent = element.parentElement;
        if (parent === null) {
            return null;
        }
        if (element.tagName == 'SPAN' && parent.isSameNode(sourcelines)) {
            return element;
        }
        return findParentSpan(parent);
    }

    //** event handler for "click" on the first line of a block */
    function lineSelectStart(e) {
        var startElement = findParentSpan(e.target);
        if (startElement === null) {
            // not a  (maybe ): abort, keeping event listener
            // registered for other click with  target
            return;
        }

        // update tooltip text
        tooltip.textContent = 'click again to terminate line block selection here';

        var startId = parseInt(startElement.id.slice(1));
        startElement.classList.add(lineSelectedCSSClass); // CSS

        // remove this event listener
        sourcelines.removeEventListener('click', lineSelectStart);

        //** event handler for "click" on the last line of the block */
        function lineSelectEnd(e) {
            var endElement = findParentSpan(e.target);
            if (endElement === null) {
                // not a  (maybe ): abort, keeping event listener
                // registered for other click with  target
                return;
            }

            // remove this event listener
            sourcelines.removeEventListener('click', lineSelectEnd);

            // hide tooltip and disable motion tracking
            tooltip.classList.add('hidden');
            sourcelines.removeEventListener('mousemove', moveAndShowTooltip);
            window.clearTimeout(tooltipTimeoutID);

            //* restore initial "tooltip" state */
            function restoreTooltip() {
                tooltip.textContent = initTooltipText;
                sourcelines.addEventListener('mousemove', moveAndShowTooltip);
            }

            // compute line range (startId, endId)
            var endId = parseInt(endElement.id.slice(1));
            if (endId == startId) {
                // clicked twice the same line, cancel and reset initial state
                // (CSS, event listener for selection start, tooltip)
                removeSelectedCSSClass();
                sourcelines.addEventListener('click', lineSelectStart);
                restoreTooltip();
                return;
            }
            var inviteElement = endElement;
            if (endId < startId) {
                var tmp = endId;
                endId = startId;
                startId = tmp;
                inviteElement = startElement;
            }

            addSelectedCSSClass(startId - 1, endId -1);  // CSS

            // append the  element to last line of the
            // selection block
            var divAndButton = followlinesBox(targetUri, startId, endId);
            var div = divAndButton[0],
                button = divAndButton[1];
            inviteElement.appendChild(div);
            // set position close to cursor (top-right)
            positionTopRight(div, e);

            //** event handler for cancelling selection */
            function cancel() {
                // remove invite box
                div.parentNode.removeChild(div);
                // restore initial event listeners
                sourcelines.addEventListener('click', lineSelectStart);
                sourcelines.removeEventListener('click', cancel);
                // remove styles on selected lines
                removeSelectedCSSClass();
                // restore tooltip element
                restoreTooltip();
            }

            // bind cancel event to click on 
        var button = document.createElement('button');
        button.textContent = 'x';
        buttonDiv.appendChild(button);
        div.appendChild(buttonDiv);

        //   
        var aDiv = document.createElement('div');
        aDiv.classList.add('followlines-link');
        aDiv.textContent = 'follow history of lines ' + fromline + ':' + toline + ':';
        var linesep = document.createElement('br');
        aDiv.appendChild(linesep);
        //     link to "ascending" followlines
        var aAsc = document.createElement('a');
        var url = targetUri + '?patch=&linerange=' + fromline + ':' + toline;
        aAsc.setAttribute('href', url);
        aAsc.textContent = 'ascending';
        aDiv.appendChild(aAsc);
        var sep = document.createTextNode(' / ');
        aDiv.appendChild(sep);
        //     link to "descending" followlines
        var aDesc = document.createElement('a');
        aDesc.setAttribute('href', url + '&descend=');
        aDesc.textContent = 'descending';
        aDiv.appendChild(aDesc);

        div.appendChild(aDiv);

        return [div, button];
    }

}, false);