Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Here is a rule about privacy between processes, and it is the heart of rush's communication model. Each process reads and writes only its own heap. Never anyone else's. So how does information travel? By the fedex. Think of mailboxes. If you want to send a letter to a friend far away, you do not go to their house. You put the letter in your own mailbox, and the mail carrier takes it. In rush, when one process reads a value from another, with something like p dot label, it is the fedex that goes to the other process, reads its struct, and writes a copy, with the sender's name, into the asker's own heap. A letter is written in the writer's own heap, and carried to the receiver's mailbox. This refusal means a process tried to reach into a foreign heap directly. The message names the door, the asker, and the source. The fix: ask through the fedex, with a qualified read or a message, and the copy will arrive in your own heap.
Text from the Rush engine's error catalogue.