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

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

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

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