public static ValueUpdate createAssign(FieldValue newValue) {
        return new AssignValueUpdate(newValue);
    }
Creates a new value update that assigns a new value, completely overwriting
the previous value.

@param newValue the value to assign
@return a ValueUpdate specifying the assignment
@throws IllegalArgumentException if the runtime type of newValue does not match the type required
document/src/main/java/com/yahoo/document/update/ValueUpdate.java