Usage:

  |fuse %dest /=kids= mate//~nel/home= meet//~zod/kids/track
  |fuse %old-desk /=kids= only-that//~nus/test=, =overwrite &
  |fuse %desk-to-cancel-fuse-into %cancel

A %fuse request in clay replaces the contents of %destination-desk
with the merge of the specified beaks according to their merge
strategies. This has no dependence on the previous state of %dest
so any commits/work there will be overwritten.

|fuse extends this concept with the idea of a tracked source. When
specifying beaks to include in your fuse, specify %track instead of
a case. This will tell |fuse to retrieve the latest version of the
source beak AND to rerun the %fuse request whenever that tracked
source changes. A fuse can have many tracked sources, or none. The
base may be tracked as well.

The overwrite flag allows you to overwrite a currently ongoing fuse.
Without this flag, attempting a fuse into a desk that you already
|fuse'd into will error.