They are defined using the command transform_aff. They can act on points, lines, circles and polygons, using the command act. They are defined by a point (the image of (0,0)) and a linear transform. In
- funct -> transform_aff Taff - funct -> point A - funct -> coord A 2.5 -0.5 - funct -> coord_aff Taff Tlin A
the first command defines the affine transform Taff. The second and third command define the point A of coordinates (2.5,-0.5). The last command sets the linear part of Taff to Tlin (the linear transform defined in 3.8.2), and the image of (0,0) to A.