do-then-else semantics improvement

[AndreVanDelft]

The behavior of do-then-else with a missing then or else clause is wrong and should be improved.
Up to now, a missing then or else clause would act as a then or else clause that behaves neutrally.

But the nature of do differs from the nature of if.
Suppose syntactically we could leave out both then and else clauses.
Then we should have the following equivalences:

if p   ===   if p then [] else []
do s   ===   do s then [+] else [-]

With other words:
a missing then would just propagate successes of the LHS upward.
a missing else would just propagate deactivations of the LHS upward

That needs to be implemented.
In principle this will be easy, but we need also new tests