"Raise" action that preserves existing expression and type

[dhess]

When we "raise" a well typed node, we lose information; i.e., we may delete one or more subtrees in the raised node, leading to a partial loss of the student's program state.

We should either modify the current raise action, or add a new action, such that the action:

1. Preserves the node's current known type.
2. Replaces the node with a hole/mismatch.
3. Inserts the original node into the hole/mismatch, annotated by the preserved type from step 1.

[dhess]

The current "raise" action is prone to losing information; i.e., it can delete nodes in the raised subexpression, leading to a partial loss of the student's program state. I've added this comment to the OP.