Bauer and Woessner [1] weren't very clear on their semantics, but according to Bruines [2], they're type annotations. [1] https://web.archive.org/web/200902200

Bauer and Woessner [1] weren't very clear on their semantics, but according to Bruines [2], they're type annotations. [1] https://web.archive.org/web/20090220012346/http://delivery.acm.org/10.1145/370000/361515/p678-bauer.pdf?key1=361515&key2=3342588511&coll=&dl=acm&CFID=15151515&CFTOKEN=6184618 https://web.archive.org/web/20090220012346/http://delivery.a... [2] http://www.cs.ru.nl/bachelorscripties/2010/Bram_Bruines___0213837___Plankalkul.pdf http://www.cs.ru.nl/bachelorscripties/2010/Bram_Bruines___02...