Regex lets wrong 7th-chords pass

[johentsch]

The specifiers M % +M can only occur in seventh chords and therefore one of 7 65 43 2 needs to follow it. Since form and figbass are defined in two separate groups, the regex matches even if figbass is missing. To improve the regex would probably require lookaheads. Or we check that elsewhere and/or do some automatic correction, just assuming 7?