Symplectic Geometry
《Symplectic Geometry》 · 第 55–56 页 · Section 3.3，all crossings are positive 的定义与 Figure 3.5 原图
If all the components of a link are oriented, each crossing in a diagram will look locally like one of those in Figure 3.5. These two types of crossing are called positive and negative. […] An oriented diagram is called positive if all its crossings are positive. […] A link is called positive if it has a positive diagram. For links with two or more components, positivity can depend on the orientations of the components.