public FieldUpdate addValueUpdate(ValueUpdate valueUpdate) {
        valueUpdate.checkCompatibility(field.getDataType()); // will throw exception
        valueUpdates.add(valueUpdate);
        return this;
    }
Adds a value update to the list of value updates.

@param valueUpdate the ValueUpdate to add
@return a reference to itself
@throws IllegalArgumentException if the data type of the value update is not equal to the data type of this field
document/src/main/java/com/yahoo/document/update/FieldUpdate.java