Merge two attributes in Shibboleth v3

[Peter Schober]

* Rod Widdowson <rdw at steadingsoftware.com> [2018-03-17 15:27]:
> You need to do some reasonably weird stuff with dependencies and
> sourceAttributeID.  Read [1] the section on Multi-Attribute
> Dependencies (and beware that this will all be deprecated in 3.4
> where the same stuff will be possible, but the code will be more
> obvious.

No. A template definition with the urn string prefix set in a
template, and a single attribute from LDAP as dependency.
That's all. Nothing weird.
-peter