PLANE MANIPULATIONS
*******************

Planes are currently generated by selecting points. Boundaries or tool
paths can be defined by tracing with the HATCH and FILL options off.
Tool paths may be animated by the alternate DRAW PLANE option.
Note that boundaries and hatchings are treated as different planes. 

The following secondary selections are allowed:-

    OPTIONS:- Not yet implemented.
    ADD:-     Planes are added by selecting points (may or may not
              be pre-defined).
    DELETE:-  Planes are deleted by locating points until the plane is
              uniquely defined.
              If option selected with the LEFT button, the user will be  
              able to delete planes in reverse order of storage.
    MOVE:-    Planes may be moved as complete entities.
              Selection of this option with the LEFT button allows for
              rotation together with translation.
    STYLE:-   Hatching lines can have line styles as for normal lines.
    CHANGE:-  Plane specifications (hatch, fill) can be altered to
              be those currently active by selecting this option 
              and then locating the plane.
              Alternate selection allows for the change of PEN NUMBER
              only, or line style and thickness only. This selection
              also allows for changes in GLOBAL mode.
    PEN:-     Specify pen number and line thickness as for lines.
    UNPAK:-   Hatchings may be unpacked into lines.
    HATCH:-   Hatchings are displayed and selected graphically. If
              hatchings are to be turned off, press the RIGHT button 
              when selection requested. IF cross-hatching is desired, 
              press the LEFT button when selecting pattern.
              If the PLANE HATCH command is selected with the LEFT 
              button, the user will be able to either alter the angle of
              hatching or to define a point through which the hatching 
              lines must pass.
    FILL:-    Similar to and can co-exist with hatching. This is the
              only part of the system that cannot be plotted out
              on a pen plotter.
              Alternate selection with the LEFT button allows for
              hatching with an existing symbol string. This can then be
              plotted on a pen plotter. Position of the original symbol
              string governs the hatching. If it is desired that this
              symbol string is not to be displayed, define it as a
              construction line item. (*WARNING* this hatching with
              symbols can be very slow for a complex boundary.)
    TRACE:-   An alternative way to add planes is to make use of the
              TRACE command. This command would allow the user to turn
              a closed polygon (may include circular arcs)
              into a plane so that it can be hatched or filled.
              The user would select a starting point. Qikdraw would
              start tracing along defined lines/arcs. Where it becomes
              ambiguous, the user would be prompted by inverse-coloured
              squares. The user would select the appropriate path by
              pointing to the point desired.
              If the TRACE command is selected with the LEFT button,
              the path can be defined to be always turning left (or
              right) whenever the path is not uniquely defined.
    MODIFY:-  Adding additional vertices to the plane. These vertices
              can only be inserted between points not linked by an arc.
              Alternate selection deletes vertices.
