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I assume the built-in definition you're referring to is [pmat](https://dfns.dyalog.com/n_pmat.htm). That illustrates how to solve your problem near the bottom. The idea is simply if σ is a permutation of length N and A is an array of length N, then A[σ] is A permuted by σ. This leads to a definition like:

```apl
permute←{⍵[pmat⊃⍴⍵]}
```

with

```apl
permute 'abc'
```

producing

```
abc
acb
bac
bca
cab
cba
```

(The example in the documentation uses `{⍵[pmat⍴⍵]}` but this seemed to lead to rank errors when I tried it in online interpreters using one of the definitions of `pmat` listed on that page.)