A set is a mathematical term used for a loose collection of objects, called elements.
They are often represented by brackets enclosing their elements (e.g., $\{1, 4, -13.5\}$).
In contrast to permutations, the order of the elements in a set is not important, so that
$\{1, 4, -13.5\}$ is the same set as $\{-13.5, 1, 4\}$. Also, the elements of a set are not
allowed to repeat (as is the case for multisets), so that $\{-1, 2.5, -1, 0\}$ does not represent a
well-defined set.
