# Circle symbol for true logical statements?

I am reading the book Introduction to Higher-Order Categorical Logic by Lambek and Scott, and have run across this inference rule when they define what they call the "conjunction calculus":

$$A \xrightarrow{\bigcirc_{A}}T$$

What does the operator $$\bigcirc_{A}$$ mean?

To quote Lambek and Scott (section 0.5) "An object $$T$$ of a category $$\scr A$$ is said to be a terminal object if for each object $$A$$ of $$\scr A$$ there is a unique arrow $$\bigcirc_A:A\to T$$."
So $$T$$ is a terminal object in a category and $$\bigcirc_A$$ is the unique arrow from $$A$$ to $$T$$.
• @Richard No, I've never seen it before. $!_A$ is pretty common. – Kevin Carlson Apr 17 at 18:27