Given a map and the apex map method putAll()
map<String,Integer> intsByKey = new map<String,Integer>
{'a' => 1, 'b' => 2};
intsByKey.putAll(new map<String,Integer>
{'c' => 3, 'd' => 4, 'a' => 10});
The documentation states that putAll replaces the contents of the target map
The new mappings from fromMap replace any mappings that the original map had.
So, how does one merge two maps in a convenient way?