procedure
                        :canvas         name

Creates a canvas widget within the current panel frame for the TCL
interpreter.
name is used to generate the path name of the window to be created. It
should have the format of a widget name  (e.g.  :canvas   CVS) and should be
unique within this interpreter. The name will be lower-cased (using the string
tolower command) in order to generate the path name used to create the
window.
Currently the canvas widget is non interactive and thus no click procedure is
invoked.
The :canvas procedure may also be used directly within a frame window (i.e. before any
panel window has been created).


The :canvas procedure may have a number of arguments however none of
these is mandatory.

        arguments
                        :x                      number
        number is the x-co-ordinate within the current panel of the canvas
widget in pixels
        e.g.    :x 400

                        :y                      number
        number is the y-co-ordinate within the current panel of the canvas
widget in pixels
        e.g.    :y 200
The :x and :y parameters may be omitted in which case the packer geometry manager will be
used and the position of the window will be determined by the contents of the master (panel)
window. If however the :x and :y parameters are supplied then the placer geometry manager
will be used to position the window within the master (panel) window.
Warning:        Note that it is not possible to mix use of the packer and placer geometry
managers within the same window. Thus all windows within a panel must request or imply that
the same geometry manager is used.

                        :w                      number
        number is the width of the canvas widget in pixels
        e.g.    :w 400

                        :h                      number
        number is the height of the canvas widget in pixels
        e.g.    :y 200

The :canvas procedure creates an empty window into which objects can be
created using the :draw procedure.