Following up on something I noticed a while ago,

        * scheme-utility.texi (Property Primitives): In primitive-property-ref,
        note parameters to not-found-proc, use hyphens rather than underscores
        for that parameter name.
        In primitive-property-set!, VAL is the value parameter not CODE.


 - Scheme Procedure: primitive-property-ref prop obj
 - C Function: scm_primitive_property_ref (prop, obj)
     Return the property PROP of OBJ.

     When no value has yet been associated with PROP and OBJ, the
     NOT-FOUND-PROC from PROP is used.  A call `(NOT-FOUND-PROC PROP
     OBJ)' is made and the result set as the property value.  If
     NOT-FOUND-PROC is `#f' then `#f' is the property value.

 - Scheme Procedure: primitive-property-set! prop obj val
 - C Function: scm_primitive_property_set_x (prop, obj, val)
     Set the property PROP of OBJ to VAL.