With ed, how do you fold and unfold lines?

Given the following file:

foo bar baz
this and that

then I can easily "unfold" those lines with global substitution with ed(1):

g/ /s//\
/g

which gives:

foo
bar
baz
this
and
that

But what's a smooth way "fold lines"? I can't use the \n or \ pattern in a g/re/s idiom. Currently I'm using the following approach: 1. Insert spaces. 2. Join.

1
+,++s/^/ /
--,.jp
++,+++s/^/ /
--,.jp
,p

to get back:

foo bar baz
this and that

but it's pretty clunky and I wonder if there's a better way!