Attribute references are used to retrieve the value of previously defined attribute for a package or project. Syntax:
attribute_reference ::= attribute_prefix ' <simple_attribute_>simple_name [ ( string_literal ) ] attribute_prefix ::= project | <project_simple_name | package_identifier | <project_>simple_name . package_identifier
If an attribute has not been specified for a given package or project, its value is the null string or the empty list.