description: |
Used to make a category be 'used' by references of the type query, selection
and masking. The property attributes of categories flagged with '@' are used
to control the behaviour of these special reference types.
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format: | @ |
example: | Sport /Ball Sports /@Soccer /@Tennis /Motor Sports /@Formula 1 p: wheels = 4 /@Motor Racing p: wheels = 2 /@Rally p: wheels = 4 Car Sports +> /Sport # wheels >= 4 % |