pub open spec fn project_first_key<K1, K2, V>(m: Map<(K1, K2), V>, k1: K1) -> Map<K2, V>Expand description
{ Map::new(|k2: K2| m.contains_key((k1, k2)), |k2: K2| m[(k1, k2)]) }Returns a new map that projects the first key of a pair (K1, K2),
keeping the values associated with the second key K2.