- "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

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.