This House would leave the European Union' is clear as it is. This House would leave Europe to itself' could be defined around EMU, the EU or the European Court of Human Rights. Sometimes a motion will be broader 'This House would look before it leaps' could question the merits of EMU or whether to buy the Spice Girls' second album. When the first speaker finishes, the grounds for debate should be clearly and precisely laid out, with no room for ambiguity - the test of a definition is whether it accomplishes this. Although it is generally accepted that the proposition have the right to define the motion in any way they choose, intelligent and straightforward definitions are expected and rewarded.
Definition is largely a matter of judgement - the following, however, are strict rules.
For example, any attempt to set a debate around the proposition that 'killing babies is wrong' is an undebatable truism. A debate cannot be defined around an area of highly specific knowledge. For example, the speaker's home village may be embroiled in controversy about the vicar's vegetable patch - unfortunately this may not be debated. Similarly, squirreling towards a pre-prepared case is not encouraged. The opposition may also choose to introduce its own positive case or counter-proposal.