Clarify the problem report for top level "let..." phrases
Currently writing "Let Cave be a room." outside a rule raises a problem report that "You wrote...but I can't find a verb here that I know how to deal with, so I am ignoring this sentence altogether."
This might be a little hard to understand for anyone who hasn't absorbed all the relevant documentation. I7 takes great pains to produce helpful messages, which is the only reason this one stands out.
A more tailored message would be splendid, a la "You wrote "let Cave..." but that suggests that Cave should be or begin a temporary name for something. I expected to see an assertion (such as 'Mr Jones is a man') rather than a variable declaration ('let Mr Jones be a man')."