support for `obtains` command, which is a variant of `have`

also somewhat generalised the `have` support altogether
use ProofEntry graph as default for now
`proof` command as a merge point
flatten parallel splits
fixed empty list exception
val order seems to be important
support for empty parallel
use Parallel soft links for merge point branches and other complex merges

Now PPTree conversion should be able to handle all graphs there and back again
forgot to mark edited methods
display parallel links in the tree