ibid or idem

Hi, I keep watching in the manual of jurabib the use of "ibid." for
repetitive uses of the same reference. Here in Spain (and in most books in
latin languages I have seen) we use "ibid." to quote the same page of the
same reference previously used.

For example:

bla bla bla bla (Pérez:1989, 25) and also bla bla bla (ibid.) ---> meaning
that both references are from the same book and page.

For repetitive uses of the same reference with diferent pages of it we use
"idem". For example:

bla bla bla bla (Pérez:1989, 25) and also bla bla bla (idem, 28) --->
meaning that both references are from the same book but different pages

I would like to know two things:

1- Why jurabib uses this format? Is it common in Germany to use it like
that?
2- How can I change "ibid." for "idem"?

Thank you very much for your kind help.

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Julio Rojas
[email protected]