- "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.