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$($define $($filter$-$credits$-$course$-$num c course$-$num num$-$credits$)$

$($local $[($define $($fn$-$for$-$course c$)$

$($cond

$[($and $(>=($course$-$number c$)$ course$-$num$)$

$(>=($course$-$credits c$)$ num$-$credits$)$

$($empty$?($course$-$dependents c$)))$

$($list $($course$-$number c$))]))$

; $($fn$-$for$-$loc $($course$-$dependents c$)))$

$($define $($fn$-$for$-$loc loc$)$

$($cond $[($empty$?$ loc$)$ empty$]$

$[$else

$($append $($fn$-$for$-$course $($first loc$))$

$($fn$-$for$-$loc $($rest loc$)))]))]$

;define helper function is a tail recursive function that use an accumulator acc to collect

;elements. The helper function processes each element in the list, updating the accumulator

;as necessary, and the recursive call is the last operation, making it tail$-$recursive.

$($define $($filter$-$helper c acc$)$

$($cond

$[($empty$?$ c$)$ acc$]$; return the accumulated result.

$[($course$?$ c$)($cons $($fn$-$for$-$course c$)$ acc$)]$ ; append new acc to

$[($not $($empty$?($course$-$dependents c$)))($filter$-$helper $($rest $($fn$-$for$-$loc $($course$-$dependents c$))))$

$($cons $($first $($fn$-$for$-$loc $($course$-$dependents c$)))$ acc$)]$; add to accumulator

$[$else $($filter$-$helper $($rest $($fn$-$for$-$loc $($course$-$dependents c$)))$ acc$)]))$ ;skip element

$($filter$-$helper c $\text{'}())))$; the above coding has bug, it cannot return course dependents are list of course, what went wrong?