úÎÀÛ$A heterogeneous map, including keys xs. A Key in a heterogeneous map. x is the key identifier type,  which ensures that we don'#t look up a key in a map that doesn't  have it. CAllocate a new key, which is only valid within the passed function 3 (and will be a type error if it tries to escape). CSame as insert, except does not add the key identifier to the map,  instead ensuring that it's already there.        hetero-map-0.21 HeteroMap.MapMapKeyempty singletonnewKeyinsertlookup overwrite:*ZInaccessreplace