#  Sofar A(n-1) n Seen
Start ← [0] 0 1 1_0_0_0
N     ← ⨬(˜-|+)⤚₂(⬚1⊡˜-)     # A(n-1) - n if valid, else A(n-1) + n.
Seen′ ← ⍜⊡⋅1⟜(⬚0↙↥⊸⊓+₁⧻)⊙⋅⋅∘ # Update Seen, expanding as needed.
Step  ← ⊃(⊃(∘|⋅⋅+₁)|Seen′)◡N # Run one step.
&p$"First fifteen terms: \n_"⍥⤚Step15 ◌ Start
&p$"First repeat: _"⋅⊙⋅◌⍢(⊸⊂⊙Step|=∩⧻⊸◴) Start
&p$"...Thinking for _ seconds..."⍜now(⍢(⊸⊂⊙Step|≠1000/+⬚0↙1000⋅⋅⋅∘) Start)
&p$"0-1000 generated at: _"⋅⋅⊙◌