apply one instruction to current goal and another to new subgoals
Major Section: PROOF-CHECKER-COMMANDS
Example: (then induct prove)RunGeneral Form: (then first-instruction &optional completion must-succeed-flg)
first-instruction, and then run completion (another
instruction) on each subgoal created by first-instruction. If
must-succeed-flg is supplied and not nil, then immediately remove
the effects of each invocation of completion that ``fails''.