So far, the theoretical class rkmmodelclass has just one attribute,
RKMMOD_CurrVal.  A couple of extra attributes can make it more useful.
Because the rkmmodel object maintains an upper limit on its RKMMOD_CurrVal
integer value, it would be useful if that upper limit was variable.  Using
a new attribute, RKMMOD_Limit, an application can tell a rkmmodel what its
upper limit is.  The rkmmodel will enforce the limit internally, so the
application doesn't have to worry about it.

Another useful addition is a pulse increment and decrement for
RKMMOD_CurrVal.  Whenever the model receives an increment or decrement
command, it increments or decrements its internal value. To make the
example class simple, rkmmodelclass implements incrementing and
decrementing by creating "dummy" attributes called RKMMOD_Up and
RKMMOD_Down.  When an rkmmodel receives an OM_SET message for one of these
attributes, it increments or decrements RKMMOD_CurrVal.  An rkmmodelclass
object does not care what the value of the RKMMOD_Up and RKMMOD_Down
attributes are, it only cares that it received an OM_UPDATE about it.

There are two pieces of data that make up this new class's instance data:
the rkmmodel's current value (RKMMOD_CurrVal) and the upper limit of the
rkmmodel (RKMMOD_Limit).  The example class consolidates them into one
structure:

    struct RKMModData {
        ULONG currval;
        ULONG vallimit;
    };