Text.PrettyPrint.HughesPJ inserting blank lines

[Christian Maeder]

I've found myself using "$+$ space $+$" in order to insert a blank line 
between two docs.

There are probably other ways to do it. "(test "")" (and $$) instead of 
"space" (or $+$) also work in many cases (but not "empty"). Which way 
would be the best?

Maybe one could add a further function "$++$" to do this and a list 
version "vsep" of "$++$"?

Another idea might be something like a vertical space "vspace" so that 
several blank lines would be created by: $$ vspace $$ vspace $$ ...

Cheers Christian