10:16 AM
is there a rational explanation for `0 1⊥n ←→ 2×n`?

@ngn Extension of ⍵
`⍞←0 0 0 1⊥123 123 123 123`

`@Adám 246`

```⎕←0 60 60⊥0 1 30 ⋄
⎕←(1,⍨⌽×\⌽1↓0 60 60)+.×0 1 30```

```@Adám
90
90```

```⎕←0 0 1⊥123 ⋄
⎕←(1,⍨⌽×\⌽1↓0 0 1)+.×123```

10:22 AM
```@Adám
246
246```

@ngn ^ ^^ etc,

13 hours later…
11:51 PM
Announcement: APLcart finally includes all* of the famous FinnAPL idiom list, fully updated to use modern APL.
* Well, except the following 13 entries, because I don't understand them. I'd much appreciate anyone's explaining one or more to me:
```BA2∧A=⌈\BA1×A←+\BA2>¯1↓0,BA2                               Groups of ones in BA2 pointed to by BA1 (or trailing parts).
BV2∧A∊(BV1∧BV2)/A←+\BV2>¯1↓0,BV2                           Groups of ones in BV2 pointed to by BV1.
⎕IO+(⍴NM)⊤-⎕IO-(,(NM=(⍴NM)⍴⌈⌿NM)∧NM=⍉(⌽⍴NM)⍴⌊/NM)/⍳×/⍴NM   Index pairs of saddle points.
(0,⍳(⍴XV)-IS)∘.+IS                                         Moving index of width IS for vector XV.
+/A∧⍉A←NV1∘.≤NV2                                           Number of ?s intersecting ?s (NV1=starts, NV2=stops).```