dynamic reference resolution

<!-- PRIMARYKEY within GLOBALS; provides value as literal -->
<PRIMARYKEY dmtype="ivoa:string" value="ICRS"/>

<!-- PRIMARYKEY within TEMPLATES; identifies VOTable column containing values -->
<PRIMARYKEY ref="_pksrcid"/>
 
<!-- FOREIGNKEY; identifies VOTable column containing values -->
<FOREIGNKEY ref="_srcid"/>

<!-- REFERENCE; direct reference to an INSTANCE -->
<REFERENCE dmref="_timesys"/>

<!-- REFERENCE; determine INSTANCE via ORM elements -->
<!--   identifies container holding options, and local FOREIGNKEYs to match against -->
<!--   this example, choose the referenced coordinate system based on current filter band value -->
<REFERENCE tableref="_CoordinateSystems">
    <FOREIGNKEY ref="_band"/>
</REFERENCE>