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In Haskell's unicode package `undefined` is the bottom symbol (⊥, UP TACK). Which is probably closer to None than the Empty set.

(However because UP TACK is a mathematical symbol, so Haskell parses it as an Operator and so it must be wrapped in brackets: `(⊥)`)

Yuck, is that a formal logic symbol? It just looks like "orthogonal" to me.

It's used as the symbol for "false" presumably because "T" stands for "true".

In a proper font its size is the same as a capital letter, and matches the symbol '⊤' for true.