The city's health ministry recently installed exercise machines at metro stations that exchange a certain number of squats for tickets.
Perhaps McInerney et al. could quantify "very difficult" with a certain probability of horizontal exchange in a given unit of time to make what counts as a public good in biology more precise.
Even a certain level of genetic exchange probably occurred between NW Africa and the South of the Iberian Peninsula [ 10, 27, 32, 33], sharp frequency changes have been described in this area [ 10, 33].
Normalizing of HIV has also been described in the very different context of gay men in North America and Western Europe [ 48- 50], where likelihood of infection is acknowledged not always with fear [ 51], in fact, there are accounts of a certain eroticization of semen exchange and possibly of infection per-se, with people actually seeking to get infected [ 52].
The dynamic credential changes its value once a user changes its location or when he has exchanged a certain number of data packets.
It was a hot summer's day - the rules back then were that you exchanged a certain amount of money into east German currency and you could not take it back out - you had to spend it.
However, while this system produces a very elegant way for disparate machines to instantly know that they are exchanging a certain class of data, it also makes it very easy for a network administrator, working with a firewall, to shut out all access.
Iraq is allowed to sell a certain amount of oil in exchange for "humanitarian" goods.
Once an exchange has a certain amount of liquidity, it is likely to attract more trading volume, even if there are more efficient, but less liquid, alternatives.
In Bursa, Turkey, some of the finest weaving ever done was by slave craftsmen, who often contracted to fulfill a certain amount of work in exchange for emancipation.
