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A constant definition is of the form
define @constant := expr;
The constant expression expr will be evalued and the constant
@constant will be defined to have this value. The constant must
not have been defined previously. The constant is valid from this
definition up to the end of the rule file. If you use the keyword
default
instead of define
, you provide a default value for
@constant. This means, the value is only preliminary and may be
changed by a normal constant definition. After a constant has been used
in an expression, its value may not be changed any more.