It could also have been both, as well. In the immediate moment it functions as an interjection and as the rest of the sentence develops it fills in a pronoun ga

It could also have been both, as well. In the immediate moment it functions as an interjection and as the rest of the sentence develops it fills in a pronoun gap. Like starting a shanty with a loud, slow single syllable to get attention (and maybe catch others on to the key) and that still being also a part of the first line of the song.