⍝ Sieve of Eratosthenes — the classic APL one-liner
⍝   primes ← (2=+⌿0=A∘.|A)/A←⍳N
⍝
⍝ Read right-to-left:
⍝   A ← ⍳N        : A is 1..N
⍝   A∘.|A         : outer-product residue table — M[i,j] = A[j] mod A[i]
⍝   0=...         : boolean — true where A[i] divides A[j]
⍝   +⌿...         : column sums — count of divisors per A[j]
⍝   2=...         : true for numbers with exactly 2 divisors (1 and self) → primes
⍝   .../A         : compress — select A[j] where mask[j] is true
⍝
⍝ Examples:
⍝   primes 10  →  2 3 5 7
⍝   primes 30  →  2 3 5 7 11 13 17 19 23 29

primes ← {(2=+⌿0=⍵∘.|⍵)/⍵←⍳⍵}