I really do think that David Cameron is unwise and silly to use House of Commons time on the subject of "We thought of the Inheritance Tax change first".
It's like a school kids' "My dad's bigger than yours" debate.
Nick Robinson has followed up the issue of Treasury minutes about the subject being considered. But, you can find government minutes to cover discussion of any subject under the sun.
Rather like the "it's political correctness gone mad" cliché , there is a newly emerging cliché which applies to the "IHT - who thought of it first?" debate: "If there is anything which puts people off politics it's this".