Suppose I have such message structure:

package msg_RepAndOpt;

message RepAndOpt{
    repeated string name = 1;
    optional string surname = 2;
    // there are lots of others.}

And I have two components that have copies of this message:

// component1:RepAndOpt A;
// component2:RepAndOpt B;

In my case components modify those messages via transaction mechanism. It 
means that if one component changes some field it also sends it to another 
component to propagate those changes. Component-receiver is doing merge:

// Component2 modifies B and sends it to component1.// Component1 perfoms merge:

Now, say, component2 wants to erase field "name". If it will send clear B 
message (default construction) than:

   - MergeFrom() will not modify A;
   - CopyFrom() will erase also other fields.

Another way will be to fill B with the contents of A, clear name field and 
component1 will use CopyFrom(). But this is not acceptable because system 
is really high-loaded and there could be lots of other fields. So, desired 
solution to clean name field is:

   1. Component2 create B message.
   2. Explicitly stores information that it want to erase only name field.
   3. Component1 perform A.MergeFrom(B).
   4. Result: A::name is cleared but other fields are left untouched.

As far as I tested this applies to repeated and optional fields. Is there 
any ready-to-use solution or I should modify protobuf implementation?

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