source f
i1 : X = abstractProjectiveSpace 4 o1 = X o1 : a flag bundle with subquotient ranks {1, 4}
i2 : f = X.StructureMap o2 = f o2 : a map to point from X
i3 : source f o3 = X o3 : a flag bundle with subquotient ranks {1, 4}