partially parenthizing a chord

[Werner LEMBERG]

Consider the following chord:

  { <a d'\harmonic a''> }

I would like to parenthesize the lower two notes of the chord.  There
should be a single parenthesis to the left and right (contrary to what
`\parenthesize` does).

Looking up the LSR I couldn't find a generic solution that isn't an
ad-hoc trickery, thus I'm grateful for further help.


    Werner