template expansion of optional variables

[lucmoreau]

Should optional variables (marked with ui:min, ui:max) be handled by the template expander?

When a variable is not filled, e.g. a plan in an association, and if the variable occurs in optional position, then the variable should be replaced by a null value.

[lucmoreau]

Should prov-template recognize "optional" positions in statements, or alternatively, should we introduce a category of variable, say varnull:xyz, which if not bound is ignored.