Why can't I access a variable named __CD__ on Windows 7?

Question

Note - this question is based on behavior observed on Windows 7. I believe the behavior applies to all other versions from Vista onward. Based on MC ND\'s answer, and Fo

Answer 1

I can't see the cause why a userdefined __cd__ is different to a user defined cd variable.
But I made some tests.

setlocal EnableDelayedExpansion
echo #0 %__cd__%
if defined __cd__ echo #1 It's defined
set __cd__=1234
echo #2 %__cd__%
echo #3 !__cd__!
call echo #4 %%__cd__%%
echo #5 %__cd__:x=y%
echo #6 !__cd__:x=y!
if defined __cd__ echo #7 It's defined
set __cd__
set /a result=__cd__
echo #7 %result%

So there are three interesting points.

1. The test with if defined is always positive
2. set can echo the user defined variable
3. set /a can access the user defined variable