The sentence *s* "In many supervised learning problems one has an output variable $y$ and a vector of input variables $x$ described via a joint probability distribution $P(x,y)$" from [wiki](https://en.wikipedia.org/wiki/Gradient_boosting)


Here implicitly declared 3 notations $\{x,y,P(x,y)\}$.


So I wonder if there is a math notation so that we can explicitly declare some notation.


For example, we can use `@`, then *s* equals to "In many supervised learning problems one has an output variable @ $y$ and a vector of input variables @ $x$ described via a joint probability distribution @ $P(x,y)$", in my personal notebook it's very handy and clear.



Is there any math notation for `be denoted by`?
We may see $:=$ used this way.
$$
\text{Let } A\_3 := \{n \in \mathbb N\;:\; 3 | n\}
$$
That is, let the set of multples of $3$ be denoted by $A\_3$.


