The methodNamespace (2nd argument) defaults to fossil.page, and
     any method-name data properties, e.g. data-f-preview-via and
     potentially data-f-preview-from/to, must be a single method name,
     not a property-access-style string. e.g. "myPreview" is legal but
     "foo.myPreview" is not (unless, of course, the method is actually
     named "foo.myPreview" (which is legal but would be
     unconventional)). All such methods are called with
     methodNamespace as their "this".

     An example...

     First an input button:

     <button id='test-preview-connector'
       data-f-preview-from='#fileedit-content-editor' // elem ID or method name

                : methodNamespace[e.dataset.fPreviewTo],
                eFrom = '#'===e.dataset.fPreviewFrom[0]
                ? document.querySelector(e.dataset.fPreviewFrom)
                : methodNamespace[e.dataset.fPreviewFrom],
                asText = +(e.dataset.fPreviewAsText || 0);
          eTo.textContent = "Fetching preview...";
          methodNamespace[e.dataset.fPreviewVia](
            (eFrom instanceof Function ? eFrom.call(methodNamespace) : eFrom.value),
            function(r){
              if(eTo instanceof Function) eTo.call(methodNamespace, r||'');
              else if(!r){
                dom.clearElement(eTo);
              }else if(asText){
                eTo.textContent = r;
              }else{
                dom.parseHtml(dom.clearElement(eTo), r);
              }