merge

auto aa1 = ["x" : 1.0, "y": 2.0];
auto aa2 = ["y":3.0];
assert( aa1.merge( aa2 ) == ["x" : 1.0, "y": 3.0] );
aa2 = ["y":3.0, "z":4.0];
assert( aa1.merge( aa2 ) == ["x" : 1.0, "y": 3.0, "z":4.0] );