Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
The engine would not start, because a builtin never answered the one question every being must answer. In rush, each being is a noun, a verb, or a flow, and it gets its kind from a single question: does running it produce a value? The builtin is supposed to answer that in a line called production_test. This builtin said nothing. And a being with no answer has no kind, which is not a state the engine allows. Think of a passport application with the nationality box left empty: the office cannot issue anything, because everything else depends on that box. If the engine let this builtin in, it could produce values while not being a verb, and that would break the central rule of the Trinity, so the engine stops at the door. The message tells you which builtin is silent. The fix is one line. Add production_test with a valid answer to that builtin's row, choosing from the answers declared in trinity.feml. Once the builtin answers, the engine can classify it, and birth goes through.
Text from the Rush engine's error catalogue.