A numbering concept key to the Bistromathics Drive. A recipriversexcluson is a number whose existence can only be defined as being anything other than itself. In other words, the given time of arrival is the one moment of time at which it is impossible that any member of the party will arrive. Recipriversexclusons now play a vital part in many branches of maths, including statistics and accountancy and also form the basic equations used to engineer the Somebody Else's Problem field.