- Expression @@ Expression -- see Expression -- the class of all expressions
- Expression @@ Holder -- see Expression -- the class of all expressions
- Expression @@ Thing -- see Expression -- the class of all expressions
- Holder @@ Expression -- see Expression -- the class of all expressions
- Holder @@ Holder -- see Expression -- the class of all expressions
- Thing @@ Expression -- see Expression -- the class of all expressions
- Function @@ Function -- composition of functions

This operator may be used as a binary operator in an expression like x@@y. The user may install binary methods for handling such expressions with code such as

X @@ Y := (x,y) -> ...

where X is the class of x and Y is the class of y.