===> -- a binary operator

Ways to use symbol ===> :

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

For the programmer

The object ===> is a keyword.

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.