Remove empty staves from PianoStaff

[Tao Cumplido]

Hi all,

I am using 2.13.39 on Windows 7.

I have a PianoStaff but I want only occasionally display both staves.
I tried \Staff \RemoveEmptyStaves but this doesn't remove empty staves from 
PianoStaff and when I put \PianoStaff \RemoveEmptyStaves LilyPond crashes when 
compiling.
I am pretty sure this has worked in previous versions, but I don't know which 
one exactly. The syntax was also a little different iirc.

I hope someone knows a solution.

Best regards,

Tao
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