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An important example is a finite geometry. For instance, in a finite plane, X is the set of points and Y is the set of lines. In a finite geometry of higher dimension, X could be the set of points and Y could be the set of subspaces of dimension one less than the dimension of the entire space (hyperplanes); or, more generally, X could be the set of all subspaces of one dimension d and Y the set of all subspaces of another dimension e, with incidence defined as containment.

An important example is a finite geometry. For instance, in a finite plane, X is the set of points and Y is the set of lines. In a finite geometry of higher dimension, X could be the set of points and Y could be the set of subspaces of dimension one less than the dimension of the entire space (hyperplanes); or, more generally, X could be the set of all subspaces of one dimension d and Y the set of all subspaces of another dimension e, with incidence defined as containment.

一个有限几何的重要例子。例如，在有限平面中, ''X'' 是点的集合, ''Y'' 是线的集合。在高维有限几何中， ''X'' 可以是点的集合， ''Y'' 可以是低于整个空间维数的一维子空间(超平面)的集合; 或者，更一般地， ''X'' 可以是一维子空间 ''d'' 的所有子空间的集合， ''Y'' 可以是另一维子空间 ''e'' 的所有子空间的集合，关联度定义也包含在内。

一个有限几何的重要例子。例如，在有限平面中,''X'' 是点的集合,''Y'' 是线的集合。在高维有限几何中，''X'' 可以是点的集合，''Y'' 可以是低于整个空间维数的一维子空间(超平面)的集合; 或者，更一般地，''X'' 可以是一维子空间 ''d'' 的所有子空间的集合，''Y'' 可以是另一维子空间 ''e'' 的所有子空间的集合，关联度定义也包含在内。

==Polytopes==

==Polytopes==