Typesetting exercises with blank space

[James Harkins]

I want to create an exercise for some beginning composition students, where 
I will provide some short melody fragments sprinkled through a longer 
score. Most measures will be empty, but I want LilyPond to leave enough 
space for the students to write in their own notes. I looked in the 
notation reference at "horizontal spacing" (which appears to be concerned 
more with note spacing) and "breaks" (where I found quite a bit about 
allowing extra breaks, which is the opposite of what I want).

I'm sure it's documented, but 1/ my mental map of the LP documentation 
isn't comprehensive enough and 2/ my brain is fried and I simply can't 
think of a reasonable search term for this at the moment.

I could imagine either:

- a setting to force a certain number of bars per system, or

- an override to set the minimum measure width.

Thanks,
hjh