backwards membership notation set theory

Question

This is more of a notational/historical question. I had a course last quarter where the professor would write things like $A \in x$ for $A$ a subset of some bigger space $X$ and $x$ an element of $X$. I thought that was a typo, but i've recently seen it again, on the 8th page (page 140) of this paper: http://individual.utoronto.ca/philipkremer/onlinepapers/DTL.pdf. I'm wondering if this is some old notation, and am curious about where it originated, if anywhere. Thanks to anyone who knows the answer to this.

Sincerely,

Vien

Answer 1

As Bageer remarks:

Formulas are inside theories and in this case the author is using $A$ as a formula and $x$ is a theory. I am pretty sure $\in$ is being used in its typical fashion.