[NTG-context] OK, I Don't Quite Understand \definefontfamily

[Michael Urban]

I don't seem quite able to put it all together, forgive the question.

I have a 'Bookman' font setup, pretty vanilla:

\definetypeface [Bookman] [rm] [serif] [bonum] [sl=name:unituscndboldoblique]
\definetypeface [Bookman] [tt] [mono]  [modern] [default]
\definetypeface [Bookman] [ss] [sans] [unitus] [default]
\definetypeface [Bookman] [mm] [math] [modern] [default]

(Unitus is the fontsite 500 'Univers' substitute, works fine)

Now, the built-in gyre bonum font doesn't have a slanted/oblique variant, so if I have Bookman as my main body
font, \sl  produces (unsurprisingly) the same result as \it, TexGyreBonum-Italic.   If I want \sl to summon
Latin Modern for the variant, I have tried variations on

\definetypeface [Bookman] [rm] [serif] [bonum] [sl=file:Latin Modern Roman Slanted]


but to no avail — not helped by the fact that the \definetypeface contextgarden page doesn't have any pointer to just what that fifth
argument can contain.


How do I accomplish this?