integer fn = open(command_line()[2],"r") -- (reading the source file)

function allx(string line)
    line[1..-1] = 'x'
    return line
end function

function longest(string mask)
object line = gets(fn)
string newmask
    if atom(line) then return mask end if
    newmask = allx(line)
    if not match(mask,newmask) then return longest(mask) end if
    mask = longest(newmask)
    if match(mask,newmask) then
        puts(1,line)
    end if
    return mask
end function

?longest("x")

close(fn)