This currently supports only DNF and CNF sentences properly.
May add support for Horn clauses later on.
Here's an enumeration of what's what:
_what_ _logical value_ string form
"empty clause" false "(False)"
"empty term" true "True"
"regular CNF" - "[(a + b)(c)]
"regular DNF" - "[a b + c]
The empty ones could've been left empty and the surrounding various brackets would tell what it is,
but this way it's quicker to remember which form defaults to what logically.