Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You called merge, and rush told you this builtin is retired. Here is why. Merge took a map and poured another map's entries into it, changing it directly. In rush, every change to a collection goes through one door, the data verbs, and direct mutation builtins were refused outright. The door for putting things into a map is data dot add. You give it the map, and then the entries you want, each with its label, a colon, and its value. For example, data dot add of m, then name colon, and a value, and color colon, and another. Every call produces the next version of the map, and the previous one stays available in time. If you have two maps to combine, add the entries of the second one into the first, using the same door. Rush says retired by name instead of not found, because it was a decision, not a typo. And if you wrote your own function called merge, yours still wins in scope. One door for all changes.
Text from the Rush engine's error catalogue.