In my first course in probability theory, our textbook was Kai Lai Chung's Elementary Probability Theory. Chung, starting from the tenth page* and continuing throughout the book, uses the notation $AB$ for $A\cap B$; the professor for the course also used this notation, and did so without comment. However, earlier today I made use of this notation while talking to a different professor, and she had never heard of it. A quick internet search following our conversation turned up no examples of Chung's notation. Thus, my question:
How common is it to denote the intersection $A\cap B$ by $AB$?
*I have the fourth edition.