Definitions give names to sketch objects. Definitions alone
are benign. A sketch input file consisting entirely of
definitions will generate no drawing. Only when definitions are
referenced do they potentially lead to ink on the drawing.
The intent of definitions is to make sketch code more concise
and readable. There is no input file employing definitions
that could not be re-written without them.
Definable objects include any result of an affine arithmetic
expression (scalar, point, vector, or transform), any drawable
object (dots, line, curve, polygon, block, sweep, put, repeat, or
special), and option strings. In addition, tag definitions,
which have no associated object at all, allow the meaning of other
definitions to be selected from a set of alternatives. Since tags may
be defined (and undefined) in the command line of sketch, they
can be an aid in the script-driven preparation of documents.