Immutable RDF graphs

I'm contemplating a problem where I want to query a single RDF graph which
derives its data from a number of changing data sources.

I could just create one memory store per source, then combine all the
stores into a 'master' store which is queried. It would have to be rebuilt
when any of the data sources changes.

Somewhere along this train of thought, I figured I want to track the
changes on each graph as they're made. It'd be nice if there was a library
for immutable RDF graphs, something along the lines of Immutable.js <
https://github.com/facebook/immutable-js>.

I venture to guess there's no such implementation that's RDF-specific. What
about graphs in general? Labeled directed graphs?

Would an implementation along these lines be a good idea, or see any demand?

Austin Wright.