Translate modifiable's.
Make pool::{is_used( ),is_free( )} public.
Add TODO notes.
Add context::get_modifiable_reg( ).
Clarify documentation.
Condense default-value setting into one place.
Remove unneeded declaration.
Remove definition of expression_ref distributed in expression.C
Choose proper default values for dirichlet.
Allow default values for lists to be set from distributions.

- Separate :~ into :~ for random variables and :=~ for data.
Update notes.