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@@ -406,11 +406,26 @@ with vectors and matrices and axis-aligned bounding box collision
detection.
@menu
+* Basics:: Commonly used, miscellaneous things.
* Vectors:: Euclidean vectors.
* Matrices:: Transformation matrices.
* Rectangles:: Axis-aligned bounding boxes.
@end menu
+@node Basics
+@subsection Basics
+
+@defvar {Scheme Variable} pi
+An essential constant for all trigonometry. @code{@U{03C0}} is the ratio
+of a circle's circumferences to its diameter. Since @code{@U{03C0}} is an
+irrational number, the @var{pi} in Chickadee is a mere floating point
+approximation that is ``good enough.''
+@end defvar
+
+@defvar {Scheme Variable} pi/2
+Half of @var{pi}.
+@end defvar
+
@node Vectors
@subsection Vectors