Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You used an old door that has been retired, and the engine is showing you the new way. The idea is simple: selection is naming. You do not call a function to pick a member, you write its name. Want the fourth item of a list? Write the list, dot, four. Positions start at one. Want the ninth, counting from zero? Write dot eight, dot base zero. Want a range, from the second to the fourth, both ends included? Write dot, then two dots between brackets. For a map, write the label: m dot age gives the value labeled age. And if the label repeats, m dot age dot two gives the second one. If you ask for something that is out of range, the answer is null, not an error. This replaces the old at call, which is gone. The message shows the retired door you used. Change it to the name form, and the line reads shorter and clearer. After all, it is naming all the way down.
Text from the Rush engine's error catalogue.