This section illustrates the use of the hitmask and memask to define one
type of collision.

Suppose there are two classes of objects that you wish to control on the
display:  ENEMYTANK and MYMISSILE.  Objects of class ENEMYTANK should be
able to pass across one another without registering any collisions.
Objects of class MYMISSILE should also be able to pass across one another
without collisions.  However, when MYMISSILE and ENEMYTANK collide, the
system should generate a call to a collision routine.

Choose a pair of collision detect bits not yet assigned within memask, one
to represent ENEMYTANK, the other to represent MYMISSILE.  You will use
the same two bits in the corresponding hitmask.  in this example, bit 1
represents ENEMYTANK objects and bit 2 represents MYMISSLE objects.

                          MeMask  HitMask
                          ------  -------
              Bit Number   2  1    2  1
              --- ------   -  -    -  -
              ENEMYTANK 1  0  1    1  0
              ENEMYTANK 2  0  1    1  0
              MYMISSILE    1  0    0  1

In the memask, bit 1 is set to indicate that this object is an enemytank.
Bit 2 is clear (zero) indicating this object is not a MYMISSILE object.
In the hitmask for enemytank objects, bit 1 is clear (zero) which means,
"I will not register collisions with other ENEMYTANK objects."  However,
bit 2 is set (one) which means, "I will register collisions with MYMISSILE
objects."

Thus when a call to docollision() occurs, for any objects that appear to
be colliding, the system ANDs the memask of one object with the hitmask of
the other object.  If there are non-zero bits present, the system will
call one of your collision routines.

In this example, suppose that the system senses a collision between
ENEMYTANK 1 and ENEMYTANK 2.  Suppose also that ENEMYTANK 1 is the
top/leftmost object of the pair.  Here is the way that the collision
testing routine performs the test to see if the system will call any
collision-handling routines:

              Bit Number             2  1
              --- ------             -  -
              ENEMYTANK 1  MeMask    0  1
              ENEMYTANK 2  HitMask   1  0

              Result of logical-AND  0  0

Therefore, the system does not call a collision routine.  But suppose that
docollision() finds an overlap between enemytank 1 and mymissile, where
MYMISSILE is the top/leftmost of the pair:

              Bit Number             2  1
              --- ------             -  -
              MYMISSILE MeMask       1  0
              ENEMYTANK 2 HitMask    1  0

              Result of logical-AND  1  0

Therefore, the system calls the collision routine at position 2 in the
table of collision-handling routines.