[tmtvl]

Okay, let me give a simpler example:

  (defun first-two (list)
    (assert (>= (length list)
                2))
    (list (first list)
          (second list)))

In a language which doesn't normalise the case of symbols you could in theory work around that by capitalising or upper-casing either the function or the variable, but that's still not a particularly elegant solution.