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The use of this notation in modal logic was developed by C.I. Lewis.

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1. (logic, unfilled) possibility, possible, possibly

   ∀𝑥□𝐹𝑥 → □∀𝑥𝐹𝑥 ⇔ ◊∃𝑥𝐹𝑥 → ∃𝑥◊𝐹𝑥 ― Barcan formula

2. (set theory) the diamond principle
3. (calculator) subtotal
4. (traffic) high-occupancy vehicle lane, bus lane
5. (botany, unfilled) sex unknown[1]

• (modal operator denoting possibility): M

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