perms←↑'ABCD' 'CABD' 'ACDB' 'DACB' 'BCDA' 'ACBD' 'ADCB' 'CDAB'
perms⍪←↑'DABC' 'BCAD' 'CADB' 'CDBA' 'CBAD' 'ABDC' 'ADBC' 'BDCA'
perms⍪←↑'DCBA' 'BACD' 'BADC' 'BDAC' 'CBDA' 'DBCA' 'DCAB'
missing perms
DBAC