: swap-char ( char1 char2 char3 -- char' )
    {
        { [ [ [ = ] 2keep ] dip 4 nrot ] [ [ 2drop ] dip ] }
        { [ [ swap ] dip [ = ] 2keep 3 nrot [ swap ] 2 ndip ] [ drop swap drop ] }
        [ 2drop ]
    } cond
;

! swaps every occurence of char1 with char2 and vice versa in string
: swap ( string char1 char2 -- string' )
    [ swap-char ] 2curry map
;