additional test cases for existential quantification
unquantified --> free
test case for group_pairs_by_key/2 with variables as keys
shorten

nf_variables/2 is not needed at all, and the property that
existentially quantified variables are always merged in the first
position is now used when deleting the corresponding track.
additional test case
shorten
update order of solutions in samples

Due to the new data structures, solutions may appear in a different order.