The call save_thm("name", th) adds the theorem th to the current
theory segment under the name name. The theorem is returned as a
value. If there is an existing theorem saved under this name for
export in the current theory,
FAILURE
Never fails.
USES
Adding theorems to the current theory. Saving theorems for retrieval
in later sessions. Binding the result of save_thm to an ML variable
makes it easy to access the theorem in the current terminal session.