A very old, very very deprecated feature. I believe pcwalton's working on removing the last vestiges of it from the compiler as we speak. The feature was called "argument modes", but it's been deprecated for so long that almost nobody remembers precisely how they were used (very few understood it in the first place). They were quite pervasive in the early 0.0 days, and it's taken this long to remove them completely.

In addition to the +foo forms, there were also others: -foo, &&foo, ++foo, perhaps more. Be glad that they're gone. :)