A fragment, when given a context and an write'able output, outputs data to the output and evaluates python code in the context.  

Example:
> l = Lexicon()
> 
> l.makeFragment((
>     "@export html-head",
>     ":<html><body>"
> ))
> 
> l.makeFragment((
>     "@export html-tail",
>     ":</body></html>"
> ))
> 
> l.makeFragment((
>     "@export hello-cells",
>     
>     "@import html-head",
>     
>     "<table>",
>     ".for y in xrange( 1, 3 ):",
>     "   :<tr>",
>     "   .for x in xrange( 1, 3 ):",
>     "       :<td>Hello << x >>, << y >></td>",
>     "   :</tr>",
>     ":</table>",
>
>     "@import html-tail"
> ))
>
> l.execFragment( 'hello_cells' )



