BFF vs first scanline, and MPEG-2 encoding

[miamicanes]

If a video is "bottom-field first", which set of fields are the "bottom" fields?

a) The EVEN fields, whose first line is on scanline 2, and whose last line presumably comes after & below the field whose first line is on scanline 1 (assumes both odd & even fields have the same number of scanlines)

b) The ODD fields, whose first line is on scanline 1, and whose last line (or partial scanline) comes after/below the last even scanline?

Put another way, if I want to bob the even fields out of existence, would I replace them with the PREVIOUS, or the FOLLOWING odd field? Likewise, if I want to bob the odd fields out of existence, would I replace them with the PREVIOUS, or the FOLLOWING even field?

[neuron2]

The lines are numbered starting from 0. So:

0-----
1 -----
2-----
3 -----

So the Top field has the even numbered lines: 0, 2, 4, ...

Your field sequence is like this with top field in uppercase and assuming BFF:

a B c D e F ...

If you wanted to replace the top field, you can do this:

a a c c e e ...

Instead of asking whether you want the previous or the next field, it's easier to just say that you copy the odd lines to the even lines in a given frame: 1 -> 0, 3 -> 2, etc.