I have two questions concerned with that is to say:
1. Does it mean exactly the same as that is? The funny thing is, CALD gives two, though quite similar, definitions: one for "that is to say" and another for "that is (to say)". Can confuse, can't it?
2. Can this phrase start a new clause (after a full stop or a semicolon)? I was pretty sure it shouldn't but have come across such a use of it.