Not Oblivion XML

The Elder Scrolls IV: Oblivion has an absolutely insane format for menu panels. It is conveniently documented by the CS wiki, but remains obtuse and difficult for humans to parse. The aim of this project is to supply a format which is both easy to read and easy to transpile to Oblivion-compatible XML.

In other words, write this:

rect name="container":
    locus: &true;
    x: 0
    y: 0
    width: 500
    # Enforce a 16:9 aspect ratio
    height: me().width / 16 * 9

    image name="icon":
        filename: some_picture\.dds
        # Set the image to use its "natural" size:
        width: me().filewidth
        height: me().fileheight
        # Position this image on the bottom-right corner of its container.
        x: parent().width - me().width
        y: parent().height - me().height

and it turns into this:

<rect name="container">
   <locus>&true;</locus>
   <x>0</x>
   <y>0</y>
   <width>500</width>
   <height> <!-- Enforce a 16:9 aspect ratio -->
      <copy src="me()" trait="width" />
      <div>16</div>
      <mult>9</mult>
      <!--
         The value is computed at run-time. This code means: 
         copy my width, divide it by 16, and multiply it by 9.
      -->
   </height>

   <image name="icon">
      <filename>some_picture.dds</filename>
      <!--
         Set the image to use its "natural" size:
      -->
      <width> <copy src="me()" trait="filewidth" /></width>
      <height><copy src="me()" trait="fileheight" /></height>
      <!--
         Position this image on the bottom-right corner of its container.
      -->
      <x>
         <copy src="parent()" trait="width" />
         <sub  src="me()"     trait="width" />
      </x>
      <y>
         <copy src="parent()" trait="height" />
         <sub  src="me()"     trait="height" />
      </y>
   </image>
</rect>

Snippet sampled from CS wiki. Note, though, that comments are not at present preserved.