This function returns a color.  SHD = C*Kr*shadow(MAP,PP);
It gives the extent the point PP is in shadow;
where 1 is completely shadowed and 0 is completely exposed.
This value multiplied by the co-efficient and the shadow color.
Refer to page 324 in the RenderMan Companion and the Pixar
PhotoRealistic RenderMan Application Node #15.

Must be a color.  This is the shadow color.
It colors the value returned from the shadow function.

Must be a float.  Defaults to 1.0.
This is the co-efficient for the shadow function.
It controls the intensity of the returned value.
For best results Kr should range from [0,1].

This must be a string. The default is empty.
The name of the shadow map file to be used.

Must be a point. The default is the surface point, P.
The surface point to be tested for shadowing.

Shadows will not be rendered on a point inside this tolerance value 
to avoid shadows that are incorrectly produced on the object that
should be casting a shadow.  Shadow bias must be set correctly or
two different types of artifacts can be produced.  If the bias is
too small self-shadowing will occur and spots of shadow will appear
on objects in the scene at inappropriate locations.  If the bias is
too large shadows will appear to be detached from objects.  A rule
of thumb for setting bias is to use 1% of the largest extent of
the objects included in the image, measured in camera space.
Setting the bias to a value greater than or equal to zero will cause
the renderer to use this bias value for all objects the shader is
used on.  Setting the bias to a value less than zero will disable
this shader-specific bias setting and allow the bias value from the
RIB file to be used.  The default value is -1 to not override the
bias value from the RIB file.

This function returns a color.  SHD = C*Kr*shadow(MAP,PP);
It gives the extent the point PP is in shadow;
where 1 is completely shadowed and 0 is completely exposed.
This value multiplied by the co-efficient and the shadow color.