rdf_keys_in_literal_map(+Map, 
+Spec, -Answer)Realises various queries on the key-set:
- all
- Unify Answer with an ordered list of all keys.
- key(+Key)
- Succeeds if Key is a key in the map and unify Answer 
with the number of values associated with the key. This provides a fast 
test of existence without fetching the possibly large associated value 
set as with rdf_find_literal_map/3.
- prefix(+Prefix)
- Unify Answer with an ordered set of all keys that have the 
given prefix. Prefix must be an atom. This call is intended 
for auto-completion in user interfaces.
- ge(+Min)
- Unify Answer with all keys that are larger or equal to the 
integer Min.
- le(+Max)
- Unify Answer with all keys that are smaller or equal to the 
integer Max.
- between(+Min, +Max)
- Unify Answer with all keys between Min and Max 
(including).