A sequence flow object class is formatted to produce the concatenation
of the areas produced by each of its children. It has a single
principal port. Its children may be inlined or displayed.
A sequence flow object is useful for specifying inherited
characteristics. For example, a sequence flow object with a
specification of a font-posture: characteristic may be
constructed for an emphasized phrase element in a paragraph.
A port of a flow object shall accept a sequence flow object if and only
if it would accept each of the flow objects in that sequence.