Aren't all election promises hypothetical?
Gordon Brown's pledge to hold a referendum on the Alternative Vote (AV) voting system - after the election, if Labour win - may partly be a bargaining chip in the event of a hung Parliament and the need to woo the Lib Dems, who have long wanted proportional representation (PR).
The trouble is that most advocates of PR don't regard AV as a proportional system, and in certain circumstances it can actually produce a less proportional result.
So I rang the Lib Dems twice last night to ask them what they thought of Gordon Brown's pledge.
"We have no comment," they told me, "about a hypothetical referendum in Labour's hypothetical fourth term."
Hypothetical? Aren't all election promises hypothetical? Lib Dem promises more especially.
Nick Clegg did tell the Guardian on Monday, however, that he would welcome a referendum on AV-plus, the more proportional variant on AV advocated by the Jenkins commission some years ago.
So an AV-plus referendum could be the obvious compromise which might see the Lib Dems backing Labour in a hung Parliament.
I stress "might".