DTV11 — regular time table is strangely constrained

In DTV Clause 17.1, in the entry for ‘regular time table’, the following Necessity appears:

If the index of some table entry2 of the time table is 1 greater than the index of some

table entry1 of the time table, then the duration from table entry1 to table entry2 is the

repeat interval of the time table.

This goes well beyond the definition, which says only that the time table has an ‘intensional definition’, i.e., a rule that determines the entries. This Necessity states one kind of scheduling rule, but it prevents the use of a rule that specifies table entries by events, or by event plus or minus duration. (Consider military time tables, which state schedules for preparatory actions relative to a planned event whose occurrence interval is not fixed. These time tables have clear intensional definitions, but they don’t have ‘repeat intervals’.)

If the Description (“A regular time table has time table entries that repeat...”) is what is intended, then the Definition is not even close to conveying that. That concept is a regular sequence of not necessarily contiguous time intervals, which are determined by a starting point and a repeat interval. If this is what is intended, this is what the Definition should say.

If the intent is as general as the definition leads one to believe, this Necessity should be deleted, or used in a Note as a pattern for a kind of intensional definition that is based on a fixed repeat interval.

The above Necessity also assumes that the time table has exactly one repeat interval, which is not itself stated as a Necessity. It should be a requirement that a regular time table has at most 1 repeat interval. If the Description is what is meant, it must have exactly one. Note also that the second “Definition” under ‘repeat interval’ should be “Possibility”.

If the definition is what is intended, there should also be a requirement that the time table has exactly 1 ‘intensional definition’, since the term is marked as an SBVR concept.

The Definition in the v1.0 specification says that the table entries “repeat according to the repeat interval”. That is the intent. A regular time table is a repeating sequence of time intervals with a fixed repeat interval. The Necessity that the issue refers to is correct. The Definition will be reworded to clarify this by eliminating ‘intensional definition’.
The term ‘repeat interval’ is confusing, because it refers to a duration, not a time interval. It will be replaced.