Add className support
validate
configure from property
async/future view loading
benchmark (w. Cells?)

cache id$el based on lifecycle?
  - better to move element out of id and into Element

SlidePanel to test nesting

set properties with attrs()?

registerElement
  - Use Context or Element
  - registerModel?

Move elements to foam.u2.tag.input ?

How to handle OnKeyInput and OnKeyTextArea
  - have two data$'s
  - set mode

ActionBorder

make template output two-pass
  - then fix tags with missing close tags

elementForFeature or elementForName needs to come out of the Context.
Are we going to have leakage?  Does it matter?
<data:feature/> (default)
<view:feature/>

same as E()

X.E('view:feature');

X.registerElement()

X.elementMap_;  -> starts with constant collection

<foam.u2.Input prop={{this.FIRST_NAME}}/>

element: true, Property property

Do we need to rename Value?

host: or $CSS shortcut


<div class="foo bar">{{.cls('foobar', function() { })}}
<div class="foo {{foobar: function() { }}}">
<div class="foo foobar{{function() { }}}">


OPTIONAL_CLOSE_TAGS
<table>
  <tr>
    <td>blah
    <td>blah
    <td>blah
  <tr>
    <td>blah
      <td>blah
        <td>blah
</table>

<div var="foo">         ???
</div>
((
  foo.cls('adf');
))


Special value:    
  $top - top most element in the template
  $e - current element


TODO:
  better name for $top?
  dynamic class syntax
  if
  repeat
  var?