The purpose of this set of commands is to permit a simple, compact display of functions and derivatives on the screen. Typically, one declares functions by using declare(f(x, y, z)) with the result that  is displayed as  (that is, only its name). One can declare many functions at once.

This scheme also displays all differentiation variables as indices and permits declaring a "prime variable". That is, for functions of one variable, derivatives with respect to that variable are displayed with a prime.

When declare is called with an algebraic expression, each function in the expression is "declared".

Note: you cannot declare for compact display known functions of the mathematical language (cos, sin, etc.) and the arguments of these functions will not be suppressed from the display when displaying unevaluated derivatives using indexed notation.

If no arguments are given to declare, the present status of the declared variables is reported. If the single argument 'prime' is given, the present status of the differentiation variable is reported.

The command undeclare accepts the same types of arguments as declare does, that is, a function, a sequence of functions, or an algebraic expression. It is used to "undo" declarations previously done by using declare.

declare and undeclare by default print informative messages when they are called. By using the extra option 'quiet' these messages are suppressed.

By default, when declare or undeclare is first loaded, the scheme for compact printing is turned ON. However, it is sometimes useful for comparison of display to toggle this feature OFF. The macros ON and OFF are provided for this purpose.

In the ON mode, the macro show displays the last output in OFF mode. This is helpful for copying expressions from the output line.

Different from macro and alias, this scheme handles only 'print/foo' subroutines. That is, it does not change input or output, only the typesetting on the screen.

These functions are part of the PDEtools package, and so they can be used in the form declare(..) and undeclare(..) only after executing the command with(PDEtools).  However, they can always be accessed through the long form of the command by using PDEtools[declare](..) or PDEtools[undeclare](..).