That's Dijkstra's later terminology. In EWD35 he introduced the operations as "passering" and "vrijgave". Then in EWD51 he uses "verhogen" and the neologism "prolagen" ("probeer te verlagen"). In EWD74 he switches to "proberen".

These documents are available from the Dijkstra archive: http://www.cs.utexas.edu/users/EWD/welcome.html