:: png-unfilter-line ( prev curr filter -- curr' )
    prev :> c
    prev 4 tail-slice :> b
    curr :> a
    curr 4 tail-slice :> x
    x length [0,b)
    filter {
        { filter-none [ drop ] }
        { filter-sub [ [| n | n x nth n a nth + 256 wrap n x set-nth ] each ] }
        { filter-up [ [| n | n x nth n b nth + 256 wrap n x set-nth ] each ] }
        { filter-average [ [| n | n x nth n a nth n b nth + 2/ + 256 wrap n x set-nth ] each ] }
        { filter-paeth [ [| n | n x nth n a nth n b nth n c nth paeth + 256 wrap n x set-nth ] each ] }
    } case 
    curr 4 tail ;

: reverse-png-filter ( lines -- byte-array )
    dup first length 0 <array> prefix
    [ { 0 0 0 } prepend ] map
    2 clump [
        first2 dup [ fourth ] [ 0 swap set-fourth ] bi
        png-unfilter-line
    ] map B{ } concat-as ;