The idea of a volume can be very abstract. Consider, for example, the set of all possible coin-flips: the set of infinite sequences of heads and tails. Assigning the volume of 1 to this space, it is clear that half of all such sequences start with heads, and half start with tails. One can slice up this volume in other ways: one can say "I don't care about the first formula_45 coin-flips; but I want the formula_32'th of them to be heads, and then I don't care about what comes after that". This can be written as the set formula_47 where formula_48 is "don't care" and formula_49 is "heads". The volume of this space is again (obviously!) one-half.