Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your call to has did not make clear which question you were asking. Let's see the two questions. The builtin has asks about membership. First question: does this map have a member under the label k? You write has, the collection, key, a colon, and the label. Second question: does this collection hold a member equal to the value v? You write has, the collection, member, a colon, and the value. They sound alike, but they are different. A map with the label apple, whose value is red, answers yes to the key apple, and no to the member apple. If you name both, or neither, the reader cannot tell which one you meant, and rush will not decide for you. So it refuses, showing the call in the first slot, and the two lawful spellings in the second. Choose the question you truly want. Is it about the labels, or about the values? Write that one, and has will give you a clear yes or no.
Text from the Rush engine's error catalogue.