Feedback from Funchal meeting on SEDS issues

[Martin Hardwick]

Dear CAX-IF Forum,

Here is the feedback from the STEP Implementors Forum on the two SEDS issues

Parameter values that are mathematically plane_angle values
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This issue has been accepted by the Part 42 committee. Text will be added
to a future versions of the standard adopting the convention proposed by the
Implementors Forum. No change is required to the EXPRESS definition of
Part 42.

However, because no change is required to the EXPRESS, the Part 42 committee
wishes to wait until more changes to Part 42 are required so that they can be
submitted together in a single Technical Change (TC) to the standard. It is 
anticipated that this will happen some time in the next 12 months.

In the mean time, the CAX IF Forums is encoruaged to use the new convention
in their implementations.

Multi-schema in one data section SEDS issue
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The Implementors Forum does not agree with this SEDS because after extended
debate and effort the STEP community has produced an ammedment to Part 21
that meets the requirements of this SEDS.

The ammendment allows a file to contain multiple data sections each of which may be
defined by a different schema. Putting the data of different schemas into different
data sections has the technical advantage of making it clear which schema owns
each instance (when an entity is defined by multiple schemas). This in turn
makes it possible to validate a section or file for conformance to its EXPRESS
definition.

The Implementors Forum recommends that CAX/IF uses the new ammendment.
An alternative solution that can be adopted with existing files is to define
a single schema containing the multiple other schemas and to define the
Part 21 file using this single schema.

The Implementors Forum will be pleased to hear back from the CAX IF Forums if 
these solutions are unacceptable. However, all changes to Part 21 are very contentious.
Extended debate will be required at the NWI, CD, DIS and FDIS stages of the ballot
process and it maybe three years before a new solution becomes available.

Martin Hardwick
Chair STEP Implementors Forum