My custom keyboard layout[1] has a third and fourth level (Alt+[Key], Alt+Shift+[Key]) containing various useful Greek and mathematical symbols, e.g. λσγδ∀∃∈⊂∞∅

My custom keyboard layout[1] has a third and fourth level (Alt+[Key], Alt+Shift+[Key]) containing various useful Greek and mathematical symbols, e.g. λσγδ∀∃∈⊂∞∅ℤℕ←→↓↑ [1]: I used Ukelele on Mac but there's also Keyboard Layout Creator for Windows and manually editing xkb files on Linux