Use of C++ container iterator with negative 1 index in X3 examples

In the Calc9 example code file vm.cpp (and in many other X3 examples) the
syntax: "stack_ptr[-1] = some_value;" is used, where stack_ptr is defined as
std::vector<int>::iterator stack_ptr.

According to the standard as I understand it, iterator indexing begins at
zero.

I can see that it compiles and executes at least for my Microsoft  Visual
Studio 2017 compiler, but is the use of a negative index valid C++ syntax?
If so what does it mean?

Thanks for any insight,
Chris


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