I'm pretty new to Semantic Mediwiki. Currently I am in the process of setting up a site  with a number of different documents types (each document type mapping to an underlying Concept or Category) .
As a way of getting started I am following the Semantic Forms Example  which creates Books and Authors and uses the property "Was written by", of type "Page" to link a Book with its Author(s).
What is the best way to create a property that constrains the link just to Authors, e.g. a property "written by", of type "Author" where only Author pages are valid for the "written by" property?