|- -- 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.

This operator may be used as a prefix unary operator in an expression like |-y. The user may install a method for handling such expressions with code such as

           |- Y := (y) -> ...

where Y is the class of y.