Next     ← ⨬(÷2|+1×3)⊸◿2
Sequence ← ˜⊂1⊢[⍢⊸Next≠₁]

⊃(↙4|↙¯4) Sequence 27
# Calculate and cache lengths (export UIUA_RECURSION_LIMIT=3000 first)
Length ← |1 memo(⨬(+1Length Next|∘)⊸=₁)
⊸Length⊢⊏⍖⊸⍚Length+1⇡99999