Major Section: PROOF-TREE
Also see proof-tree and see stop-proof-tree. Note that
:start-proof-tree works by removing 'proof-tree from the
inhibit-output-lst; see set-inhibit-output-lst.
Explanations of proof tree displays may be found elsewhere: see proof-tree.
In a nutshell: :start-proof-tree causes proof tree display to be turned
on, once it has been turned off by :stop-proof-tree.
Do not attempt to invoke start-proof-tree during an interrupt in the
middle of a proof.
