code: @Override
    public VType addTo(KType key, VType incrementValue)
    {
      return putOrAdd(key, incrementValue, incrementValue);
    }
docs: Adds <code>incrementValue</code> to any existing value for the given <code>key</code>
  or inserts <code>incrementValue</code> if <code>key</code> did not previously exist.
  
  @param key The key of the value to adjust.
  @param incrementValue The value to put or add to the existing value if <code>key</code> exists.
  @return Returns the current value associated with <code>key</code> (after changes).