Currently there is no way to specify a constraint on Duration such that both of the following can be specified at once:
the allowed units are specified, e.g. the current PWD constraint allows only weeks and days
the time limit, e.g. the current |P0W..P50W| allows this
This constraint is needed at least for the common duration constraint for pregnancy.
The existing syntax strings must continue to parse as they do now:
Example can be found online in this test archetype: http://www.openehr.org/wsvn/knowledge/archetypes/dev/adl/test/basics/adl-test-ENTRY.basic_types.v1.adl?op=file&rev=0&sc=0 (see duration_attribute_101 - 207)