[Rd] Remove env var with .Renviron?

[Duncan Murdoch]

I have two .Renviron files:  one in my home directory, one in the 
current directory.  From my reading of ?Startup, I thought the local one 
would override the home dir one, and if I set a value, it does. 
However, if the local .Renviron contains

RGL_DEBUGGING=

while the home one contains

RGL_DEBUGGING=true

I see that the env var is set to "true" when I start R.  I need to set 
the local one to

RGL_DEBUGGING=""

to set a zero-length value. Is this expected?

Duncan Murdoch