add-expression-source-element
add-expression-source-element

This patch adds the expression source-element type, and changes all the
expression source-elements to inherit from expression instead of inheriting from
source-element directly.  I've also changed the types for a few slots from
source-element to expression where appropriate.

darcs-hash:20060409014434-53808-a21c04528decce6353f146cd5bd294749ce933d2

chumsley/jwacs,chumsley/jwacs,chumsley/jwacs