operand _

An unknown object is an object without any sub-objects meaning no content. An operand _ is an unknown object without symbol as no name. In the same expression within "(" and ")", different operand _ is different unknown object. After the expression including operand _ resolving, there is a collection of possible satisfied combinations of operand _ s gotten. If more than one operand _ are needed to appoint to the same unknown object, the only way is using the named unknown object assigned with ∅ to replace the operand _ s. If there is no space among the operand _ and another characters, what matched is not an object, it is the object symbol characters match. However, each symbol and character is an object too.