[gjm11]

Interesting, but given that the code in question computes (expt 2 488) as the square of (expt 2 244), is there actually any way for that bug to cause the observed symptoms?

(2^244 is being shown as zero. 2^488 is being shown as non-zero; presumably the right value though I haven't actually checked. If 2^244 is actually being computed as zero, then unless what's happening somehow depends on the depth of the stack -- surely not possible if this bug is the cause -- 2^488 should be computed as zero squared, which is zero.)

[wingo]

Sure, it's possible. The sequence of multiplications chosen to reach the exponent is different, so these expressions take different paths through scm_integer_expt.

[gjm11]

But the sequence of multiplications chosen to reach the exponent isn't different. What the user's expt function does, when asked to compute (expt 2 488), is to compute (expt 2 244) and then square it. When it computes (expt 2 244) it does the exact same sequence of multiplications as if you just asked for that directly from the REPL.

What am I missing here?