So, that deep red line - the immigration cap - is scored into today's "The Coalition: Our Programme for Government" [475KB PDF] document:
The next sentence is the interesting one, though:
It is the "mechanism" that is going to be the really tricky bit.
At the moment, the only non-EU economic migrants admitted into the UK are those with enormous wealth, enormous brains or with specific skills in areas where Britain has an identified shortage.
How will the mechanism prevent the cap damaging the national interest? There are already warnings that stopping people coming to the UK who have skills or investment we need would undermine another of the coalition's stated aims:
As revealed on this blog, official data show that the number of non-EU economic migrants employed in the UK is falling - down 76,000 last year compared with the year before. Some sectors of the economy are already complaining that they cannot fill key vacancies.
The cap could only apply to Tier 1 and Tier 2 of the existing points-based system, since those are the only two categories under which migrant workers from outside the EU can come to the UK.
Tier 1 is for "Highly skilled workers, investors and entrepreneurs". It is hard to imagine that these are the kind of immigrants the UK would want to ban.
Tier 2 covers "Sponsored skilled workers", mostly defined as "people coming to the UK with a skilled job offer to fill a gap in the workforce that cannot be filled by a settled worker". Again, it is difficult to conceive how, in the short-term, stopping these individuals would be good for Britain.
The Migration Advisory Committee (MAC) was set up to offer "independent, transparent and evidence-based advice to government on labour market shortages that can sensibly be filled by migration". Only non-EU workers with a job and the right skills in a sector identified by the committee are allowed in.
Some British business leaders are already fuming that the MAC has not agreed to put their sectors on the list of skill shortages which would allow them to bring talent in from overseas. The suggestion that we need a cap seems to imply that the government believes the committee has not been tough enough.
So it is going to be interesting to see how the cap "mechanism" might work: set the limit high and there's no point in having it; set it low and Britain deprives itself of workers which benefit the UK. The thinking cap will be worn.