st: what does Stata do when I type "var1==var2==0"?

["Hau Chyi" <hauchyi@gmail.com>]

Dear listers,

I have 4 observations for two variables that look like this:
var1 var2
0    0
0    1
1    0
1    0

when I type:
-assert var1==var2==1-
Stata returns:
-3 contradictions in 4 observations-

I realize I need to type:
-assert var1==1 & var2==1- to get the right answer.

I have a vague idea as to why Stata prefers the second one. But what's
intrinsically wrong with the first one?

Thanks in advance!

Hau
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