Prolog terms represent a few of the documented C structures. Most of
the documented C structures are not needed in ProXL, for example the
XWindowChanges structure. The documented structures that are
represented in ProXL by Prolog terms are XPoint, XSegment, XRectangle,
XArc, and XColor. The functor of these terms is the structure name
translated by the naming conventions above. The first four kinds of
terms have the same arguments as the corresponding C struct members,
except that the angles in an arc specification are given in degrees,
rather than 64ths of a degree, as in Xlib. Color terms have only the
red, green, and blue components, and these are given as numbers between
0 and 1. For example, a 30 by 40 pixel rectangle at location (10, 20)
would be represented by the term rectangle(10,20,30,40), and the color
magenta would be represented by color(1,0,1). See the section of the
ProXL manual on colors for information.