What is this semi-circular symbol in the middle of a formula?

Question

I have tried every search term I can think of but I can find no way of knowing what this symbol between the (a) and the trP means:

Some context here, Point 2 in the third paragraph. Original image from a lecture on CSP, so can't link to that one specifically, but I'm pretty sure this is in the same context.

Answer 1

I found it used in Communicating Sequentional Processes, by C. A. R. Hoare. In it, he defines the symbol as (page xvii)

(between traces)
followed by

$$\text{For example, } \langle a,b,c \rangle=\langle a,b \rangle \text{ (symbol) } \langle \rangle \text{ (symbol) } \langle c \rangle$$