Case and string?

I'm having trouble figuring out how to evaluate a string using a TinyScheme (case ) construct. I am consistently getting wrong results.

In the 3 attached test cases (no pun intended), I try different tacks to get the case to match arbitrary strings.

The goal is to return a results string.

In the first file, I code case as follows:

(case (substring testString 0 3)
  ("RGB" "Found RGB")
  ("CW-" "Found CW-")
  ("CCW" "Found CCW")
  (else (string-append "Didn't find anything in '" (substring testString 0 3) "'."))
)

Running test case 1 returns "Found RGB" no matter what I use for the test string.

In the second file:

(case (substring testString 0 3)
  (("RGB") "Found RGB")
  (("CW-") "Found CW-")
  (("CCW") "Found CCW")
  (else (string-append "Didn't find anything in '" (substring testString 0 3) "'."))
)

Running test case 2 returns "Didn't find anything in '[first 3 chars of the input]'." no matter what I use for the test string.

In the third file, based on some Scheme code I found with a Google search:

(case (string->symbol (substring testString 0 3))
  (('"RGB") "Found RGB")
  (('"CW-") "Found CW-")
  (('"CCW") "Found CCW")
  (else (string-append "Didn't find anything in '" (substring testString 0 3) "'."))
)

Running test case 3 also returns "Didn't find anything in '[first 3 chars of the input]'." no matter what I use for the test string.

Is there a way to accomplish string compare via (case ), or will I need to do this using (if ) statements? (I have multiple sets of strings to check for, several of them more than 3 items, so I'd like to use (case ) if it's doable.