- 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.