What is the meaning of the symbol \stackrel {<}{\neq} (or \lneqq)? In the book Good Math by Mark C Chu-Carrol there is the following formula on page 130:
$$
x ∈ (A ∪ B) \lneqq x∈ A ∨ x ∈ B
$$
I don't know the name or meaning of this symbol and I can't seem to find it anywhere.
In the book it says the formula is a formal notation of set union.
So what does $$\lneqq$$ mean?
 A: author here.
It's a typesetting error. It's supposed to be the if-and-only-if symbol, which looks like <=>. 
I'm not sure why it was printed that way; in my pre-prints, it printed correctly. Getting the math stuff printing correctly in the book was a huge hassle - my publisher hadn't printed anything that had non-trivial math notation before.
I'm sorry that this slipped by. Hope knowing what it's supposed to look like helps!
A: I reiterate my previous deleted answer.
I've browsed your book [Mark Chu-Carroll, Good Math A Geek's Guide to the Beauty of Numbers Logic and Computation (2013)] until around page 150.
The symbols  $\stackrel {<}{\neq}$ is "clearly" used in the context of set opeartions, like : union, intersection and cartesian product, which usually are written as (e.g.union) :

$x ∈ (A ∪ B) \Leftrightarrow x∈ A ∨ x ∈ B$.

The symbol "works" as a sort of "definitional equality" in the meta-language, like the symbol "$=_{def}$".
I was not able to find in the first 150 pages, its definition/explanation ...
