FEATURE: Missing atoms

[codereport]

These are the major things missing from Jelly(fish):

Major

• Uiua's ⊜ (partition) modifier -> part & part_with
  • note that Uiua's ⊜ is a mask cut and part & part_with are predicate cuts
• Q's prior (a 2-wise reduce)
• BQN's ⌾ (under)
  • although maybe there is no real need because everything is a single character atom
• Uiua's ▽ (keep) / BQN's / (replicate)

Minor

• BQN's ⊒ (occurrence count)
• [ ] Dyalog APL's ≠ (unique mask)
  • Turns out Jelly has this "ŒQ" (distinct sieve)
• Q maxs
• FEATURE: Q differ #7