Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You gave a builtin a word it does not know, and the engine is telling you which words it does. Some builtins accept only one word from a closed list. Assert accepts words like equal, not equal, greater, less, true, and false. The http builtin accepts get, post, put, patch and delete. The date builtin accepts of, format and parse. Web sockets accept open, accept, receive and send. Each list lives in the builtin's own declaration in builtins.feml, and the worker checks your word before the body runs. In the past, an unknown word was reported as a failed assertion, saying five against six, which had nothing to do with what you really did wrong. Now the message is honest. It shows the word you wrote, and every legal word. So if you wrote get with a typo, you can see right away what the correct spelling was. Pick one of the legal words, and the builtin runs as intended. A closed list makes mistakes impossible to hide.
Text from the Rush engine's error catalogue.