adda's subfile support

7.17: adda/subfile support:
. adda can use adde ml to define subfiles
and can also treat any of html's
hierarchical structures as subfiles .
[7.27: intro:
. the idea of subfiles as chunks of text,
is to have an documents arranged like
folder trees of files
without depending on the platform's
native folder and file system .
. files usually contain several subfiles,
and in this way act as folders .
. conversely, a folder of files
may be identified as one document .
. how subfiles are packed into files
depends on how the subfiles are used:
documents that are frequently modified
will be packed into more files .
. very large doc's will be packed into
more folders .
. adde ml (markup language) is a
simplified form of html that uses
an exclamation point in brackets
as an escape for text about text,
such as identifying the title of a subfile .]