Help on Item/Grob creation

[Carl Sorensen]

I've (somewhat) successfully created a FretDiagrams context, with a
Fret_diagram_engraver.  I can sort of make things work.  Now I'd like
some clarification on Items.

As part of the engraver, I have the code

fret_diagram_ = make_item ("FretDiagram",SCM_EOL); //WHAT GOES HERE?

It appears to me that the "name" of the Item, FretDiagram, is in fact a
pointer into scm/define-grobs.scm.  Since I want to make a FretDiagram
Item, I believe I need to make an alist entry for FretDiagram in
scm/define-grobs.scm.  Is there any documentation for what goes in the
alist entry?

Also, as part of making the item, I need to have a "cause", which is
SCM_EOL in this particular example.  It appears from some of the
internal documentation that the cause is a music expression, generally
the one responsible for creating the Item/Grob.  What is the cause used
for?

Also, what is the difference between an Item and a Grob?  From some
earlier descriptions, I think that an Item is one type of Grob; that is,
all Items are Grobs, but not all Grobs are Items (i.e. some Grobs can be
Spanners, etc.).  Is this correct?

Thanks,

Carl