Yes, of course, we've thought about it. Equally importantly, we have a "satisfies" keyword for stating checks about properties. (See the example on the Pyret home page.) We use test oracles extensively in teaching; see these two assignments: http://cs.brown.edu/courses/cs019/2012/assignments/sortacle , http://cs.brown.edu/courses/cs019/2012/assignments/oracle .

This exists in Haskell's doctest library

    -- | Documentation goes here
    --
    -- > 2 + 2 == 4
    -- True
    --
    -- prop> \lst -> reverse (reverse lst) == lst