[Ur] Toy relational algebra interpreter

[Artyom Shalkhakov]

Hello all,

I wrote a toy relational algebra interpreter a while back:

https://github.com/ashalkhakov/urweb-misc

In particular, you can take a look at the test:

https://github.com/ashalkhakov/urweb-misc/blob/master/tests/rel.ur

(It works!)

Highlights:

* relation variables are directly acessible in Ur/Web
* the usual relational algebra operators are given precise static types

At runtime, this is extremely inefficient: relvars are implemented
using plain singly linked lists, there is no indexing at all.

I'm still wondering how to properly perform a parallel renaming
operation (i.e., renaming of multiple fields of a record
simultaneously).

-- 
Cheers,
Artyom Shalkhakov