Well, for starters,

    start env = function
      | [] -> ()
      | line :: lines
        when String.length line > 4 && String.sub line 0 3 = "+++" ->
          header env line;
          modified env 0 lines
      | _ :: lines -> start env lines

is a bit elitist way to write a 2 argument function as a combination of 1 argument function and a lambda function. So let me rewrite:

    start (env) (list) = match list with
      | [] -> ()
      | line :: lines
          when String.length line > 4 && String.sub line 0 3 = "+++" ->
          header env line;
          modified env 0 lines
      | _ :: lines -> start env lines

So the function `start` takes some kind of state `env` as a first argument, and then it takes a list as a second argument, here named `list`.

If the list is empty, [], it does nothing.

If the first list element, which is a string, starts with "+++", it calls two other functions: first header(env,line) where `line` is now the first list element, and then modified(env,0,lines) where `lines` is the rest of the list, not containing the first element.

If the first list element does not start with +++, it recurses to the rest of the list.

So this is not a filter: After first encounter of "+++" it calls those two other functions and stops.