Element of a Singleton (set with one element) notation I was wondering what the notations are for indicating the element of a singleton (or unit set, or set with cardinality 1).  This would be the inverse of set construction:
$$X = \{y\} \tag{1}$$
$$y = \text{? } X \text{ ?} \tag{2}$$
I haven't seen examples of it, but I think using a notation like $X_1$ or $X_0$ is misleading.  The general case of $X$ may not even be countable, even though it is obviously countable when $|X|=1$.  For example, if $M$ is a set of sets of real numbers:
$$\forall X \in M \,:\,|X|=1 \Rightarrow P(X_0) \tag{3}$$
This seems like a possibility but since all of the $X$ aren't countable it looks misleading.
I found this post that used a notation
$$y = \iota X \tag{4}$$
Linguistically it seems similar to the English article "the".  I would probably read the above as "y equals the X".
I don't know how commonly used or recognized that notation is.  Are there any other notations, possibly more common?

If you want to be strict with set theoretic context, then $y=\bigcup\{y\}=\bigcup X$. But this might not work very well outside of set theoretic contexts.
In the case that $X$ is a subset of an ordered set, then $y=\min X=\max X$ as well.  There's probably no good, and general notation for this. But I honestly don't see why we would need one.
