Prime minister's questions sketch
By Nick Assinder
Political correspondent, BBC News website
Here's a lesson in undermining your opponents that might have graced Machiavelli's "The Prince".
It might even have come under that book's heading "Avoiding Flatterers".
Mr Clegg ignored the prime minister's trap door
In this case it was Gordon Brown's approach to the new Liberal Democrat leader, Nick Clegg.
Mr Clegg was already facing the difficult decision about how to approach these weekly sessions.
Should he go for the Cable approach and attempt to dominate his opponent with some devastating put downs?
Or should he try the Ashdown line and pick a good, solid Liberal issue to offer some heavyweight credentials.
And, whichever one he chose, would he be able to pull it off.
The first was always going to be a minefield. Following Killer Cable was the equivalent of taking the stage after Frank Sinatra, or perhaps more appropriately, Jack Dee.
The danger in the Ashdown approach was it would lack immediate impact in favour of building a weightier image.
In the end he went for the latter with a worthy question about energy bills for the poor and old. All good and well, and he did himself no harm at all.
Mr Brown welcomed the new Liberal Democrat leader
But it was the "welcome" he was given by Mr Brown that may have threatened to do some damage.
The prime minister said he looked forward to working with him on many of the issues facing the country. And he added that, as he had told Mr Clegg in their private conversations, there was an "open door" for him in Downing Street.
It seemed harmless enough. But then, what sort of impact might it have had amongst Lib Dem MPs or party workers not to mention those wavering Tory voters Mr Clegg is desperate to attract.
You could almost hear the whispers along the backbenches: "what private conversations, what are they up to, is Brown offering him a job?"
Paranoia perhaps. But paranoia is a powerful weapon. And, of course, deals and even hints of deals between Labour and the Liberal Democrats have caused nothing but grief for the party and its leaders in the past.
This was exactly the same tactic Mr Brown attempted against Mr Clegg's predecessor, Sir Menzies Campbell, with no joy whatsoever.
Sir Menzies snapped back that the prime minister's open door looked like a trap door to him.
Mr Clegg ignored the trap and ploughed ahead with his line of questioning.
Again, there was no great downside to that and he will be happy he was not mauled, he didn't get into the Punch and Judy he apparently abhors (where have we heard that before?) and he indicated the sort of serious image he wants to project.
But will he ever drop into Downing Street for tea and biscuits with Mr Brown - don't bet on it.