(ref.doc)Harel88

David Harel (Note:
 Harel)
On visual formalisms
article in Communications of the ACM
May 1988, Volume 31, number 5, p 514-530

p 515
[...] a way of representing a collection of sets, together with
some structural relationships between them.

p 515
[...] it is often desirable to identify the Cartesian product of
some of the sets.

higraphs, statecharts
blobs and edges (or hyperedges)

p 516
[...] with this "unique-contour" convention, the only real,
identifiable sets are the atomic sets [...]

p 520
semantic, or associative, networks
Semantic networks can be thought as concept-relationship
diagrams [...]

p 522
Indeed, people working on the design of really complex systems
have all but given up on the use of conventional FSMs and their
state diagrams for several reasons:
(1) State diagrams are "flat". They provide no natural notion of
depth, hierarchy, or modularity, and therefore do not support
stepwise refinement, top-down, or bottom-up development.
(2) State diagrams are uneconomical when it comes to
transitions. An event that causes the very sane transition from
a large number of states, such as a high-level interrupt, must
be attached to to each of them separately, resulting in an
unnecessary multitude of arrows.
(3) State diagrams are extremely uneconomical, indeed quite
infeasible, when it comes to states [...]. As the system under
description grows linearly, the number of states grows
exponentially [...]
(4) Finally, state diagrams are inherently sequential in nature
and do not catter for concurrency in a natural way.

p 527
[set inclusion (inheritance) and set membership (containment)]
We have chosen to adopt the former for as the meaning for blob
enclosure [...]

p 528
One can provide reasonable definitions of connectivity,
transitive closure, planarity and acyclicity in higraphs

p 528
The main thesis underlying this paper is that the intricate
nature of a variety of computer-related systems and situations
can, and in our opinion should, be represented by visual
formalisms: visual, because they are to be generated,
comprehended, and communicated by humans; and formal, because
they are to be manipulated, maintained, and analysed by
computers.

Personal note: no support in this notation for nested types...