This is an error in the latest unstable WebDSL editor. This is new behaviour; used to work fine:

entity Alias {
key :: String (id)
name :: String (name, index(10), collation(binary), searchable)
persons -> Set (inverse=Person.aliases)
aliases -> Set (inverse=Alias.aliases) // related spellings of the same name
}

I get the following error:

Inverse annotations cannot be declared on both sides. Remove either the (inverse=Alias.aliases) or the (inverse=Alias.aliases) annotation
person-data.app /researchr/person line 206

Submitted by Eelco Visser on 21 May 2011 at 17:02

The inverse anno check was strengthenth to check for properties across extend entities; apparently this also introduced this behavior. Should be fixed with rev. 4676.