How to switch from single staff to double?

[Bob Kline]

I'd like to set a song starting with the version in unison, which should
print a single staff for a couple of lines (systems), then on the third
line switch to the SATB chorus on two staves.  How do I do that, please?
 I've tried \RemoveStaffEmptyContext, but that doesn't get rid of the
extra staff on the first line.

Thanks!

Bob Kline