  When are all positions of a game numbers? We show that two properties are
necessary and sufficient. These properties are consequences of that, in a
number, it is not an advantage to be the first player. One of these properties
implies the other. However, checking for one or the other, rather than just
one, can often be accomplished by only looking at the positions on the `board'.
If the stronger property holds for all positions, then the values are integers.
