Haskell FSM - odd number of a's

The following machine accepts C-style strings with a representing any non-special character, Q representing a double-quote mark, and B representing a backslash (\). Thus the string "It's \"here\"!" would be written "QaaaaaBQaaaaBQaQ".

fsm with four states
This is implemented by the Haskell functions isString.hs. Be sure to read the comments in that file!

Testing isString: 

    ghci> :load isString.hs
    [1 of 1] Compiling Main             ( isString.hs, interpreted )
    Ok, one module loaded.
    ghci> isString "QaQ"
    True
    ghci> isString "QaQa"
    False
    ghci> isString "QaQQa"
    False
    ghci> isString "QaBQa"
    False
    ghci> isString "QaBQaQ"
    True

An alternative implementation based on using patterns with function arguments is isStringWithPatterns.hs This version will crash on a string that is not in the set ('a' | 'Q' | 'b')*.