function onFold(cm, from, to) {
        var state = cm.state.foldGutter;
        updateFoldInfo(cm, from.line, from.line + 1);
    }
Triggered when a code segment is folded.
@param {!CodeMirror} cm the CodeMirror instance for the active editor
@param {!Object} from  the ch and line position that designates the start of the region
@param {!Object} to the ch and line position that designates the end of the region