Quoting from the CASL Tools Summary [Too97]:
Use: for exchanging parsed CASL specifications or units (i.e. incomplete specifications).Informal description: An interchange format defines a portable representation of CASL, allowing CASL documents to be transmitted between different products or machines. This is a textual representation of Abstract Syntax Trees. The interchange format should support attributes to encode layout information and other information which is not considered to be part of the structure of a CASL specification. It should support also the possibility to be dynamically enriched by new attributes. The proposed format is SGML text with some indexing method.
It would be straightforward to provide an SGML document type definition (DTD) corresponding exactly to the abstract syntax of CASL [LD97], with a markup tag for each constructor of the abstract syntax, e.g., a total-function-type construct would be marked up in the Interchange Format as
where the ... provides the marked-up argument and result sorts. If desired, the markup could be minimized, so that various opening and closing tags may be left implicit when determined by the context, e.g., </total-fun-type> could be implied by </fun-decl>. Clsoing tags may in any case be abbreviated to </>.<total-fun-type>...</total-fun-type>
Less clear is how best to represent comments, annotations, and layout: