Autocompletion in the repl

[euanlacy]

I have been looking at the completion functionality of rusty-line, and reckon I might be able to get something working if people are interested in autocompletion of predicates in the repl.

[mthom]

Please do! I'd love that.

[triska]

Autocompletion is now available, and I find it extremely useful, thank you a lot!