DIF                                                                Menu: SETOP
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Subtracts the interpenetrating portion of one solid entity from another,
creating a new solid entity.

        1. (IND) solid from which to subtract
        2. (Y/N) confirm choice
        3. (IND) solid to be subtracted
        4. (Y/N) confirm choice
        5. (Y/N) do you want to keep the second entity?
        6. (Y/N) keep the result ?

Description:
This command subtracts one interpenetrating entity from another.
Parameters 1 and 2 ask you to identify the first entity and confirm your
choice.  Parameters 3 and 4 do the same for the second entity.  Parameter 5
asks you whether you would like to keep the subtracting entity as well as the
subtracted entity.

If the subtracting entity is kept it remains a separate entity (i.e., it is
not fused as in the \uni command). The system will display the resultant
entity and ask you (parameter 6) if you wish to save it.

The result of this process is a complex solid entity (data type 9). If this
command is executed on an axial wall, the result will be a complex wall that
continues to possess wall characteristics but the commands \chaw, \chaa,
\fixw, and \fixa no longer function.
        Example: \dif (ind) y (ind) y n y
        Result:  The second entity is subtracted from the first entity and
                 the result is kept.
Limitations
1. It is recommended to execute \dif on entities of the same color otherwise
   unpredictable results may occur.
2. When two entities exist in different layers, the resulting solid will
   reside in the layer of the first entity.
3. If \dif is executed on two entities that are not in contact with each other
   they may either be united into a single complex entity or the subtracting
   entity will be removed, depending on the option selected.
4. The number of \dif commands that may be executed on any entity is limited
   to a maximum of 200 points on any one of its polygons. A message will
   appear if you go beyond this limit.
5. It is not possible to execute \dif on negative entities.
6. This command will not operate on placed entities that contain negative
   solids or 2D entities.
