remarks about ACD syntax

[ableasby at hgmp.mrc.ac.uk]

Guy,

It is possible to use the values of list datatypes. As an
example you could look at the use of 'type' in
prophecy.acd

There is the attribute 'nullok'  (as in   nullok: Y)  to
specify that an outfile needn't be created.

I suppose we could extend this attribute to other datatypes
to get around the default requirement but haven't yet
found the need. Adding a default value isn't that much
effort after all.

Cheers
Alan