start env [] = env
    start env (line:lines)
        | length line > 4 && take 3 line == "+++" =
            modified (header env line) 0 lines
        | otherwise =
            start env lines

or even

    start env [] = env
    start env (line@('+':'+':'+':_:_):lines) =
        modified (header env line) 0 lines
    start env (_:lines) =
        start env lines

?