  This is a resubmission of preprint 9401008 , which has some TeXnical errors
introduced by the "reform" procedure (designed to avoid precisely these
problems!). The original can be formatted by editing out the messages "%%
following line cannot be broken..." at line 1674. This problem has been
corrected in the current version. The content of the paper has not been changed
in any way. My aplogies for any inconvenience caused. SB
