Jot

The glyph jot is a small circle represented by the unicode character ∘, "ring operator" (◦, "white bullet", may also be used in some dialects). Derived from the Iverson notation convention of using a circle for the top (reducing) function of an Inner Product to turn it into an Outer Product, the jot now has several uses in various dialects:

• In APL\360 and all other APLs, it is a "special" left operand to Inner Product (.), so that (∘.f) is an Outer Product by f.
• In SHARP APL, ∘ is the enclosed empty numeric vector, which may also be written <⍳0.
• In NARS, Dyalog APL, and other nested APLs derived from these, ∘ is the Compose operator.
• In NARS and NARS2000, for an array a, a∘/ is Mask and a∘\ is Mesh.

Thus jot may be considered either a special element of syntax as in APL\360, an array as in SHARP APL, or an operator. In the last case it would not normally be allowed as an operand to the Inner Product operator, and a special rule is needed to allow the Outer Product syntax. As a final option, APL2 allows a jot only in the context of an Outer Product like APL\360, but documents that jot "is treated syntactically as a function."