Mark part of datatype mapping text as non-normative

[kleihan]

Section 3.6.3 Datatype mappings has the following text:

Rather than creating numerous one-off mappings to other type systems, is it likely more efficient to map each to ISO 11404 once, and then compare mappings in an associative model to identify the commonalities. (The XML schema standard claims to derive its datatypes from ISO 11404, but mapping to the generic standard is more precise and comprehensive than filtering it through the XML interpretation.)

This suggestion should be marked non-normative. Furthermore, we intend to develop a standard for mapping.

This change was initially proposed by @donmendelson.