tremolo confusion; a three-bar tremolo between two whole notes

[Kenneth Wolcott]

Hi;

  I'm still confused by two-note tremolos.

  I have a piece which I am engraving from as a sample (see attached
screenshot) that has a three-line tremolo connecting two whole notes,
in 4/4.

I am looking at the documentation
(https://lilypond.org/doc/v2.22/Documentation/notation-big-page.html)
and I'm not getting how to get from this where I need to go.  I have a
two-line tremolo but not a three-line tremolo.

Here's my code snippet:

  \repeat tremolo 8 { d,16 d }  | % m57

I have another screenshot showing my engraving of these two measures.

Thanks in advance for clearing up my confusion.

Thanks,
Ken Wolcott

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