  Given a strong homotopy pushout cube of spaces A, we measure how far it is
from also being a homotopy pullback cube. Explicitly, letting P be the homotopy
colimit of the diagram obtained from A by forgetting the initial vertex
$A_\varnothing$, we study the homotopy fibre of the double suspension of the
comparison map $A_\varnothing \to P$. This difference is expressible in terms
of the homotopy fibres of the original maps in A.
