Fix assertion.
Only change modifiables in terminal contexts.
Move reg_is_shared_with_parent(,)
Check that mappings match at root, and all copies have vm_relative=0.
Actually remove computations when clearing and unsharing them.
Remove context::computation_for_reg( ) and context::result_for_reg( ).
Don't e.g. access results unless we are root.
Add assert.
Decrement the version when we change a reg's value!
Remove local computations when changing a reg, if there are any.
Remove unshare_and_clear( ).