: depermutation ( seq -- n seq ) dup natural-sort [ [ index ] curry map >factoradic factoradic>integer ] keep ;
: factoradic>integer ( factoradic -- n ) <reversed> [ factorial * ] map-index sum ;
: >factoradic ( permutation -- factoradic ) dup [ unclip [ '[ _ over < [ 1 - ] when ] map ] keep ] replicate nip ;

usage:

( scratchpad ) "abcd" depermutation [ 1 + ] dip permutation >string .
"abdc"