Is there any formal notation for dealing with lists, rather than sets?
e.g. if I have a set $X=\{x_1,\dots,x_n\}$ and I want to add a new item to the set, say $x_{n+1}$, I can say "Let $X = X \cup \{x_{n+1}\}$" and it is clearly understood that I want to add $x_{n+1}$ to my set.
However, if $X$ is not a set but rather a list, or tuple (i.e. the elements are ordered and duplicates are allowed), is there any way of indicating that I am adding an element to the end of the list?
e.g. given $X=(x_1,\dots,x_n)$, how do I say add an element to $X$ such that $X=(x_1,\dots,x_n,x_{n+1})$? i.e. how do I formally denote appending an element to $X$?