Frank, were you planning to put in low-level support in the galley stuff
for vertical spacing to be handled as "base-to-base" distance? Since
that is what real designers use, I think it should be the default
interpretation of higher-level parameter values. But "extra space" specs
are also needed at least in the case where the second object in a
sequence does not have a normal baseline (e.g., a graphic following some
text).