- 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`.