Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You wrote process dot start, and rush told you that this verb does not exist. Here is the correct way to begin a process. A process in rush is explicit: you mint it with process dot new, or with process dot spawn. Then, if you have a declared function and want it to run on that process, you say process dot spawn with run, a colon, and the function call. That is the whole vocabulary. Process dot start was never part of the design. The rule is plain: it does not exist. Rush could have said not found, but that would sound like a typo, and not the retired word that it is. So it names the ruling and the replacement. Suppose you have a function called worker. Write process dot spawn, run colon worker with its brackets, and your function walks on its own process. If you defined your own function named process dot start in scope, yours still wins. Use spawn, and your process is born.
Text from the Rush engine's error catalogue.