Above, except that if the last line of the first argument stops at least one position before the first line of the second begins, these two lines are overlapped. For example:
> text "hi" $$ nest 5 (text "there")
lays out as
> hi there
$$ is associative, with identity empty, and also satisfies
* (x $$ y) <> z = x $$ (y <> z), if y non-empty.