ID $=$ 'identifier'

HEADLINE $=$ 'headline'

LISTED $=$ 'listed'

FEATURES $=$ 'features'

LOC $=$ 'location'

DESC $=$ 'description'

END $=$ 'ENDOFLISTING'

SEP $=\text{'}$;$\text{'}$

# We store locations as tuples of two strs: $($city$,$ country$).$

LocType $=$ tuple$[$str$,$ str$]$

# ListingValueType is the type for valid values in the ListingType dict.

# All values are None or str$,$ except for the value associated with key LOC,

# which is LocType, or key FEATURES, which is list$[$str$].$

ListingValueType $=$ None $|$ str $|$ LocType $|$ list$[$str$]$

# ListingType is a dict that maps keys ID$,$ HEADLINE, LISTED, FEATURES,

# LOC, and DESC to their values $($of type ListingValueType$).$

ListingType $=$ dict$[$str$,$ ListingValueType$]$

# BookSysType is a dict that maps listing identifiers to listings,

# i$.$e$.$ to values of type ListingType.

BookSysType $=$ dict$[$str$,$ ListingType$]$