LINE MANIPULATIONS
******************

    OPTIONS:- The following line options are available:-
                  Rectangular box;
                  Lines tangent to circles (internal or external);
                  Extending/shrinking two lines to meet at a point;
                  Trimming one line with respect to another;
                  Perpendiculars from points to (or away from) reference 
                      line/arc;
                  Bisecting angles or lines;
                  Line with a bend and terminating arrow;
                  Swapping the direction definition of a line.
    ADD:-     Lines may be added in SNAP or NOSNaP (see MODE) mode.
              If the Key-Board is selected as the primary entry device, 
              the user can type in co-ordinates of the end points 
              defining the lines. The co-ordinates are absolute by 
              default. However, the input may be prefixed by the 
              characters M,R or P (or any of their combinations). M 
              means move, R means relative & P means polar (distance,
              angle in degrees).
              If the "LINE ADD" command has been selected by pressing 
              the LEFT button, locations selected will either snap to 
              line/line intersectn points or vertices of symbols,
              wherever the tolerance is not exceeded.
    DELETE:-  Lines may be deleted by locating the pointer close to the 
              line to be deleted. Be careful to avoid possible ambiguity. 
              If selected with the LEFT button, the user will be offered 
              the option to delete all construction lines or else
              delete lines in reverse order of storage.
    STRETCH:- A line can be stretched along its own length without 
              altering its orientation. If selected with LEFT button,
              option to extend lines to infinity.
    STYLE:-   Various line styles can be specified.
              Line styles with embedded symbols are also supported.
              If selected with the LEFT PUCK button, the length of repeat
              of non-solid lines can be specified.
    CHANGE:-  Line specifications can be altered by selecting this 
              option and then locating the line to be changed. The 
              specifications (pen, thickness, style, terminators) for 
              the line will then be changed to those currently defined.
              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:-     The user will be prompted for a pen number. 
              The tertiary menu for line thickness would then be 
              displayed, and a selection requested.
              CONSTRUCTION LINES are those lines that will not appear on 
              a DRAW ALL (or final plot) command.
    MERGE:-   Merge two lines (end points must be co-linear).
              Alternate selection allows for merging a block
              of lines.
    //:-      Lines may be specified parallel to a reference line at
              prescribed distances or passing through arbitary locations. 
              Alternate selection allows for generation of a pair of 
              lines at prescribed off-set (start & end) from the 
              reference line (open or closed at ends).
    BREAK:-   Points lying on a line does not imply that the line has 
              been divided into segments. This option will divide the 
              line into segments according to the number of points lying 
              on the line.
              Alternate selection with the LEFT button allows the 
              erasure of part of the line. This portion may be specified 
              as centred between the end points of the line. 
    ALIGN:-   This option allows the alignment of lines. The line to be 
              changed will be made parallel to and at a specified 
              distance from the reference line. The new end points will 
              be the projections of the old end points onto the new line. 
              OOPS if LEFT button pressed.
              If option selected with the LEFT button, the old end pts 
              will be moved to new positions without altering connected 
              entities.
    TERMI:-   Terminators can be specified for a line. This is very much 
              more efficient then specifying them separately.
    ANGLE:-   This will force the lines added to be at a specific angle 
              (or multiples there-of). Thus, setting angle to 90 would 
              cause all lines to be orthogonal except when snapping to 
              an existing point. The constraint can be relative to the 
              X-axis or a direction defined by a line.
