⍬ — Represents an empty numeric vector or zero plus tilde (hence zilde) — Keystroke Alt + Shift + } — Character ⎕UCS '⍬' = 9068
Symbolically zero or 0 represents number while tilde or ∼ represents not. Together the overstrike zilde symbol or ⍬ means not a number, i.e. empty numeric vector. APL frequently uses overstrike characters in dual-combined meaning, thus ⍞ which is ⎕ or prompt and the single quote symbol yielding prompt for string or character input versus ⎕ or prompt for numeric input.
The above examples for infinity and negative infinity correctly run in NARS2000. In theory ONLY - ⍬ would be hypothetically equivalent to ⍳0 or 0⍴N (where N is any number) since zilde or ⍬ represents an empty numeric vector.