Consider using UNICODE NULL symbol '␀' for nulls

[pathikrit]

Char nullSymbol = '␀'

[stanch]

While the semantics appeal to me, I am not convinced, because this symbol is somewhat hard to read in comparison to ∅...

[pathikrit]

Maybe an option override it then? RefTree.Null(displayAs = "␀"). I can imagine people may want to display it as string "NULL".