IF the EU's Amsterdam summit in June is likely to be remembered for any landmark constitutional innovation it will be for the introduction of formal procedures for what is known variously as "flexibility", "enhanced co operation", or "differentiated integration".
Although a mouthful of the latter formula is probably the technically best description for the proposed structures that will allow some member states to press ahead faster inside the EU with co operation in areas where other partners fear to tread - but it will be known to one and all as "flexibility".
This week foreign ministers got their teeth into the issue for the first time.
The challenge is to find the means whereby an enlarged Union, possibly up to 27 members, is not forced by the dead hand of unanimity voting to move only at the snail's pace of its least enthusiastic or least economically able member. The German Foreign Minister, Mr Klaus Kinkel, flexibility's most ardent advocate, this week described the likely result as the means by which the Union would meet the twin but seemingly contradictory challenges of simultaneous enlargement and deepening integration.
The French and Germans are particularly keen to ensure that projects like the Schengen Accord for passport free travel are developed within the framework of the Union rather than outside.
Flexibility has been used before by the Union, but in the Maastricht Treaty only ad hoc arrangements were provided for to allow for British opt outs on social policy - in the view of the substantial majority, not a success - and, with the Danes, an opt out on the single currency. The idea now is to provide at the Amsterdam summit in June clear treaty rules for future such developments.
In the Irish draft treaty, prepared for the Dublin summit, the issue was touched on largely only in general terms although provision was made for two specific limited cases - in the field of security policy it was proposed to create a system of "constructive abstention" whereby member states opposed to participating in a mission could abstain and not be involved, rather than block the whole project by voting against.
And the draft suggests that some joint conventions could come into force in memberstates once they ratified by them, rather than waiting for full ratification by all member states.
But wider institutionalised flexibility poses a number of dangers to the cohesion of the Union, most importantly in its potential for creating two or more tiers of unequal members, effectively handing a greater influence to an advance guard of inner core states. Some member states, Ireland included, although accepting its inevitability, worry that it will change the character of the Union and are determined to build in stringent safeguards.
Speaking to journalists in Strasbourg last week, the Taoiseach, Mr Bruton, said he had yet to be convinced on the need for a general flexibility provision and put down a clear marker that Ireland would be seeking to ensure that the rules would only be used in very exceptional circumstances and that a substantial majority of member states will have to want to participate.
Britain and Denmark go further in insisting that any request to create a new area of reinforced co operation should require unanimity. London sugars the pill by saying it would be willing to given political assurances that such a veto would only be used in exceptional cases when the move threatened vital interests, whether its own or those of the whole Union.
But memories of Britain's non co operation policy over BSE are fresh, and most argue that even a theoretical veto would undermine the whole idea by leaving the Union hostage to individual states willing to blackmail others over unrelated issues. The name of Greece crops ups regularly in discussion.
But while unanimity question is certain to be the thorniest problem, a host of other challenges face negotiators:
. Whether to establish one procedure, or one in each of the three decision making pillars of the Union the latter is likely;
. How to finance new structures - the Commission suggests it should bear administrative costs while participating states bear operational costs;
. Whether the Commission's sole right to initiate proposals in areas like the single market should be extended to the provisions for flexibility - the small countries bare likely to insist on this as the best protection for the already agreed common policies;
. Whether flexibility provisions should only apply to areas where unanimity currently prevails - probably;
. How to protect the interests of non participating states;
. How to keep structures open to future participation of all "willing and able" - who defines "able" in each case will be a substantial problem;
. How voting arrangements will work - will all member states still attend all meetings even if they do not vote when not participating in new structures;
. Whether national commissioners be excluded from Commission discussions of issues in which their member state is not a participant - almost certainly not, as the Commission is seen as a collective unit.
What is clear is that the issue will not be resolved until the very last stages of the Inter Governmental Conference process in June or later.