Engraver_group with parent context type?

[Lukas-Fabian Moser]

Folks,

while trying to tidy up ly/engraver-init.ly a bit (which seems to have 
experienced some rank growth over the years) I stumbled upon:

\context {
  \Staff
  \type "Engraver_group"
  \name "DrumStaff"
  \alias "Staff"

  % (etc.)

}

That's the only instance of a context definition that contains _both_ a 
parent context type (\Staff) _and_ \type "Engraver_group".

Is that correct? I don't know enough about the inner workings to judge 
reliably.

Lukas