main(): int b ref int a = b REF_HOLD_ID (left don't have reference, right is ID) int c = 2 ID_VAL a = c REF_ID (left has reference, right is ID) a = 4 REF_VAL (left has reference, right is register or value) ref int d = a REF_HOLD_REF (left don't have reference, right is a reference) ref int e = c e = d REF_REF (left has reference, right is reference) int f = c ID_ID int g = c ID_REF