Allow for definition inheritance

[J5lx]

It would be nice to be able to create definitions which inherit from other definitions, so something like

<definition id="def">
    <!-- Lots of code
     |
     |
     |
     |
     | -->
</definition>
<definition id="other_def">
    <!-- The whole s**tload of code again
     |
     |
     |
     |
     | 
     And then, one little change -->
</definition>

could be shortened like so:

<definition id="def">
    <!-- Lots of code
     |
     |
     |
     |
     | -->
</definition>
<definition id="other_def" use_def="def">
    <!-- Only differing code -->
</definition>

Let me know what you think!