Codemirror, create a new document in the same editor

What is is correct process for creating a new document (or switching documents) in Code Mirror so that the document history is not transferred to new document?

When I set the content of a new document and then do an undo, the content of the file is replaced with the content of the previous file.

Currently my code that switches files looks something like this:

$('#file-list').on('click', 'li', function () { 
    var el = $(this),
        filename = el.data('file'),
        f = files[filename];
    $('#theme-file-active li').removeClass('active');
    el.addClass('active');
    var doc = cm.getDoc();
    cm.swapDoc(doc);
    doc.clearHistory();
    cm.setValue (f.content);
    doc.setHistory(f.history);
});