Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You wrote push, and rush told you that no push survives. Here is the history. Push was the classic way to add an item at the end of a list. Many languages have it, and so did earlier versions of rush. Then came a clear decision: no push survives. Everything that changes a collection is a data verb, one family of doors, with one grammar. That means adding is data dot add. You give it the list, and the value, and you get the next version of the list, with the value at the end. The earlier version is never destroyed. It may feel like a small change in spelling, but it means one habit covers every kind of change, from adding to removing to replacing, and your data keeps its full history. Rush says retired by name, instead of not found, so you know this is a rule and not a typo. If you wrote your own function called push, it still wins in scope. Replace push with data dot add, and carry on.
Text from the Rush engine's error catalogue.