Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You used remove dot at, and rush told you that it has been retired. Let's talk about what changed. Removing something from a collection is a change, and in rush all changes go through a single door: data dot something. Direct mutation builtins were refused outright. So remove dot at is gone, and its replacement is data dot remove. You give it the collection, and then at, a colon, and the position or the label. Positions start at one, not zero. For a map, you can use the label instead. For instance, data dot remove of xs, at one, takes out the first item and gives you the next version of the list. By the way, for a while this word was refused with the message of a different retirement, the old at door, which named the wrong reason. That is fixed now, and the message points at the right ruling. If you defined your own function called remove dot at, yours still wins. Use the door, name the place, and you are done.
Text from the Rush engine's error catalogue.