[prove].
'Command examples:      CHECK|: prove x>0 by contradiction.'.
'                       CHECK|: prove c#1 by implication.'.
'                       CHECK|: prove c#3 by induction.'.
'                       CHECK|: prove c#2 by cases.'.
'                       CHECK|: prove x+y>=0 by cases on h#4.'.
''.
'The "prove ... by ..." command allows you to employ any of the four proof'.
'strategies illustrated above on a conclusion or formula.  You will enter'.
'a new proof frame in which the conclusions are what must be proved to'.
'meet the goals of the selected proof strategy.  There may also be new'.
'hypotheses which hold in this proof frame, again added temporarily while'.
'you are inside this proof attempt.'.
