- Expression and Expression -- see Expression -- the class of all expressions
- Expression and Holder -- see Expression -- the class of all expressions
- Expression and Thing -- see Expression -- the class of all expressions
- Holder and Expression -- see Expression -- the class of all expressions
- Holder and Holder -- see Expression -- the class of all expressions
- Thing and Expression -- see Expression -- the class of all expressions

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

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

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