I just realized I had a typo in my membership test and was worried this bug had been causing issues for a while. However, the code had behaved just as expected. Example: "test" in "testing" in "testing" in "testing"  This left me wondering how this membership expression works and why it's allowed. I tried applying some order of operations logic to it with parentheses but that just breaks the expression. And the docs don't mention anything about chaining. Is there a practical use case for this I am just not aware of?
in is a comparison operator. As described at the top of the section in the docs you linked to, all comparison operators can be chained:  Formally, if a, b, c, …, y, z are expressions and op1, op2, …, opN are comparison operators, then a op1 b op2 c ... y opN z is equivalent to a op1 b and b op2 c and ... y opN z, except that each expression is evaluated at most once.  So: "test" in "testing" in "testing" in "testing"  Is equivalent to: "test" in "testing" and "testing" in "testing" and "testing" in "testing" 