Steel Bank Common Lisp

Short answer is that arguments are currently only available to the
stepper for full calls. This is not a fundamental limitation, but we
don't currently store information that would allow the stepper to
figure out the arguments for calls implemented directly using machine
instructions.

If you want to see value TEE there, add

  (declaim (notinline -))

to the function, which prohibits it from being open coded.

Cheers,

 -- Nikodemus