[SCL] two comments

[John F. Sowa]

Murray, Tanel, et al.,

The distinction between an abstract and a concrete syntax
is a very precise, formal distinction.  There is no such
thing as multiple levels of this distinction.  The basic
idea is that all semantic definitions are stated in terms
of an abstract syntax, which is not printable, and that same
abstract syntax can be realized in multiple concrete forms.

In that sense, KIF, CGIF, CLML, etc., are different concrete
syntaxes for one and the same abstract syntax.  The is no
sense in saying that any concrete syntax is more or less
concrete than any other.

>I think you've quite succinctly pointed out that what is at one level
>abstract is at another concrete. These various levels of "meta-ness"
>are there up and down the levels of description, and unavoidable. The
>thing to do is be sure to keep the levels clear and distinct.

As Pat pointed out, it is a notion that was originally
proposed by John McCarthy.  It was taken up by the formal
specification community and made into the foundation for the
Vienna Definition Language (VDL), which evolved into the
Vienna Definition Methodology (VDM).

The ISO Abstract Syntax Notation ASN.1 echoes some of the
terminology, but there is much more behind the methodology
than just notation.

For a history of the development and a cast of characters
involved see

   http://www.cs.ncl.ac.uk/research/pubs/articles/papers/470.pdf

John Sowa