[dhanvanthri]

I mean it's literally the same idea as in set theory.

phi is an empty set,

(phi) is a set containing an empty set

((phi)) is a set containing a set containing an empty set

Furthermore, it's one of the only syntax rules; if it's not a list, then the first element is an operator/function.

[Koshkin]

Hmm, never thought of ∅ as a φ. (According to Wikipedia, the symbol was introduced in the 1930s by Weil, inspired by the letter Ø in the Danish and Norwegian alphabets.)

[Rerarom]

I agree with that, but in set theory the symbol {} is used.

I have nothing against it, I just think it should be emphasized more.