Base -- get the base variety of a flag bundle

i1 : X = abstractProjectiveSpace 4

o1 = X

o1 : a flag bundle with subquotient ranks {1, 4}

i2 : X.Base

o2 = point

o2 : an abstract variety of dimension 0

i3 : Y = abstractProjectiveSpace_3 X

o3 = Y

o3 : a flag bundle with subquotient ranks {1, 3}

i4 : Y.Base

o4 = X

o4 : a flag bundle with subquotient ranks {1, 4}