Can/May drafting (was Re: Question regarding DR 09-0216)

[rjelliffe at allette.com.au]

I think "may" is probably the correct usage here.

The "permissive" verbs should be used with regard to conformance (both
inclusion and exclusion.)

The "possibility and capability" verbs should be used for things outside
the control of technology being standardized.

If we have a standard for dog's shoes, then "A dog can have three legs"
would be correct but "A dog may have three legs" would be incorrect usage:
the dog is not the subject of the standard but an external fact.

For example, if we were to express the rules for XSD's schemaLocation
hint, we would have

"Namespace URLs accepted by systems *can* be any LEIRI. For conformance,
the namespace URL *may* be any IRI, but *shall* not have any spaces
encoded directly."

In this example, we have a difference between

 1) can- what the external technology could throw up
 2) may - what the allowed technology is for conformance
 3) must - hard constraints regardless.

Cheers
Rick

> In the context of standardization, modal verbs (such as "can" and "may")
> should be used with care just like instructions to computer programs, so
> that non-native speakers are not confused.
>
> The rest of this mail is extracted from Annex H of "ISO/IEC
> Directives, Part 2:  Rules for the structure and drafting of
> International Standards".
>
> The verbal forms shown in Table H.3 shall be used to indicate a course
> of action permissible within the limits of the document.
>
> Table H.3 Permission
>
> -------------------------------------------------------
> |Verbal form
> |      Equivalent expressions for use in exceptional cases
> |      (see  6.6.1.3)
> |
> |-------------------------------------------------------
> |may
> |  is permitted
> |  is allowed
> |  is permissible
> |-------------------------------------------------------
> |need not
> |  it is not required that
> |  no $B!D(B is required
> |-------------------------------------------------------
> |Do not use "possible" or "impossible" in this context.
> |Do not use "can" instead of "may" in this context.
> |
> |NOTE 1 "May" signifies permission expressed by the document, whereas
> |"can" refers to the ability of a user of the document or to a
> |possibility open to him/her.
> |
> |NOTE 2 The French verb "pouvoir" can indicate both permission and
> |possibility.  For clarity, the use of other expressions is advisable
> |if otherwise there is a risk of misunderstanding.
> |-------------------------------------------------------
>
> The verbal forms shown in Table H.4 shall be used for statements of
> possibility and capability, whether material, physical or causal.
>
>
> Table H.4  Possibility and capability
> |---------------------------------------------------------
> |Verbal form
> |   Equivalent expressions for use in exceptional cases (see 6.6.1.3)
> |
> |---------------------------------------------------------
> |can
> |  be able to
> |  there is a possibility of
> |  it is possible to
> |---------------------------------------------------------
> |cannot  be unable to
> |  there is no possibility of
> |  it is not possible to
> |---------------------------------------------------------
> |NOTE     See Notes 1 and 2 to Table H.3.
> |---------------------------------------------------------
>
>
> Regards,
>
> SC34/WG4 Convenor
> MURATA Makoto (FAMILY Given)
>
>