That version of `atom?` always returns nil, so I'll offer a correction.
Here's a very direct transliteration:
(= atom?
(fn (x)
(and (no (acons x)) (no (no x)))))
The equivalents for `not` and `null?` in Arc are both `no`, because the empty list is the only falsy value. Scheme uses dedicated values for true and false, `t` and `#f`, but Arc uses the symbols `t` and `nil`, and `nil` doubles as the empty list.
Here's a version that uses the utilities Arc provides out of the box:
