Merge `self` with another flow set.

Property definitions

	# Merge `self` with another flow set.
	fun flow_union(o: SELF): SELF is abstract

src/saf/saf_base.nit:96,2--97,42

	redef fun flow_union(o) do return new FlowHashSet[E].from(union(o))

src/saf/saf_base.nit:127,2--68