negative array index?

Hello, all!

i've seen the following construct in several pieces of C code, and i
can't for the life of me figure out (A) what it is and (B) why it is
legal:

.... myarray[-1] ...

Can someone please explain to me what it means to have a negative
array index?

Example snippets, taken from the SpiderMonkey source code:

stephan@[EMAIL PROTECTED]
> grep -e '\[-1' *.*

js.c:        value[-1] = '=';
....
jsstr.c:    argv[-1] = STRING_TO_JSVAL(str);


If someone could explain this phenomenon to me i could certainly sleep
better at night.

:)
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