Trivial solution for trivial goals.

[luke-clifton]

The [Test] passed to the solve function is a conjuction of tests.
In a similar vein to all, an empty list should mean that the condition
was met. That is, all the conditions in the list were met, there just
weren't any conditions.

This comes up when you want to create a problem which is attempting to
find a plan between two [a]s. The [] to [] case will result in a
failure to solve due to no goals being set.

[luke-clifton]

I raised this more to start discussion than actually proposing a way to do this. Perhaps there just needs to be a shift in the way the input is fed to downward when there are no goals.