public String getPrevious(String current) throws IllegalArgumentException {
        if (!containsGroup(current)) {
            throw new IllegalArgumentException();
        }

        if (this.groups.indexOf(current) == 0) {
            return null;
        }

        return this.groups.get(this.groups.indexOf(current) - 1);
    }
Gets the group before the group provided

@param current the group after the group being requested
@return the group name, or null if the start of the track has been reached
@throws IllegalArgumentException if the track does not contain the group given
common/src/main/java/me/lucko/luckperms/common/model/Track.java