Why only one custom field (of type) per parent?

[Cambell Prince]

Hi,

In the lift standard and rng I see this:

    <sch:rule context="field" >
      <sch:assert test="not(preceding-sibling::field[@type=current()/@type])">
        Fields should have different types.
        There is only one field with a given type allowed in any parent element.
      </sch:assert>
    </sch:rule>

Why do we have this?

I'm running someone's data through solid to export to lift and they have this:

\lx
  \sn
  ...
  \au digger.wav
  \au digger2.wav

(where au indicates audio)

Seems to me that it would be reasonable to have multiple recordings of the same thing.  For the purpose of this discussion say I choose to put these into lift as custom fields then would this be acceptable (currently it isn't)

            <field
                type="rawaudio">
                <form
                    lang="zx-zxxx-x-audio">
                    <text>digger.wav</text>
                </form>
            </field>
            <field
                type="rawaudio">
                <form
                    lang="zx-zxxx-x-audio">
                    <text>digger2.wav</text>
                </form>
            </field>

Regards,
Cambell