Interface for the most common Expression Languages
Strategy for putting an expression with needs to be different for those two cases:
- PublicSharing means that this language supports the creation and sharing of Expressions, which is the common use-case
- ReadOnlyLanguage means that the Language implements a pre-defined set of expressions (which can be infinite or finite). For example the url-iframe Language which directly maps URLs to addresses - meaning every well formed URL is an address in this Language. Or a potential Language implementing the verbs/predicates of a spec like FOAF.
Returns an Expression by address, or null if there is no Expression with that given address