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Feature - class to represent OpenType feature-definition record
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CONSTRUCTOR:
Feature() - generic constructor, creates an empty Feature record
Feature(Feature) - copy constructor
Feature(string tag) - creates feature, assigns 'tag' and empty value
Feature(string tag, string value) - creates feature and assigns values to both attributes
ATTRIBUTES:
parent (read-only) - parent object, Font
tag (string) - feature's tag
value (string) - feature's text
OPERATIONS:
METHODS