如何画一个箭头
首先要了解一个概念：「path object」。
Path objects are objects that are formed with lines or curves, examples of path objects include polygons and polygonal lines, splines, circles and ellipses, rectangles and circular arcs.
By clicking near the ends of the segment shown in the path properties field, you can toggle the front and rear arrows. Only polygonal lines, splines, and circular arcs can have arrows.
如何画一个节点
这里「节点」指的是图论中的点（vertex/node）。在 Ipe 中可以用 Marks 表示一个节点，我们可以调整节点的类型、大小、颜色等属性。
节点的颜色分为线条颜色（stroke color）和填充色（fill color），需要注意的是
The mark shapes "disk" and "square" also use only the stroke color. You can make bicolored marks using the mark shapes "fdisk" and "fsquare".
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