Ok thank you for the feedback, I get the message not to re-purpose currently valid syntax.

For that reason, this won't fly:

I see no technical issues in allowing
something like `[ x, y || ys]`

`||` is already an operator in the Prelude.

> [x, y ,: ys ] -- ? not currently valid

We could make list syntax work harder

> [x, y ,:+ ys ]

Means desugar the commas by applying constructor `:+` instead of `:`. That could be in general any constructor starting `:`.

Or indeed could be a pattern synonym starting `:`, which is a 'smart constructor' to build the list maintaining some invariant.