Any code that's not tested has bugs. As for reading code, when I read yours I can't work out why you put

  .filter{l, i -> i < 1 || lines[i - 1] != 'a'}

instead of

  .filter{l, i -> i < 1 || lines[i - 1][0] != 'a'}

to skip the line after one that begins with an 'a', or

  .filter{l, i -> i < 1 || lines[i - 1] != "a"}

to skip the line after one that is an "a". I'd need to go run it to work out what it does.