To sort or arrange according to characteristic or class.
To furnish with, or make up of, various sorts or a variety of goods.
To be associated with; to consort with.
To be of a kind with, to harmonise or match.
To sort or arrange beforehand.
To order (goods or services) in advance, before they are available.
A binary relation that is reflexive and transitive.
An order for goods or services placed in advance.
Such that, recursively, the root is visited before the left and right subtrees.