Updated is better than Upded as a variable name.
Performing updates in Relev data type.

Before we can detect if a particular Relev has a value, we need to
replace it. It kind of works as the eval, but I should probably improve
it in the future.
Trying to implement the beta-as-relevance for let

Regular let might need a fresh beta per binding. This is my attempt at
implementing this. Code looks ok, output does not.
Improvements in the let expressions.

First part of my attempt to implement the code for the let expressions
as described in the new text.