bug 7248: differentiation bug in D(1,z)

(94) -> D(sin(z)^2+cos(z)^2, z)

   (94)  0

(98) -> D(simplify(sin(z)^2+cos(z)^2), z)

   (98)  0

BUT

(95) -> D(1, z)

   There are 5 exposed and 0 unexposed library operations named D
      having 2 argument(s) but none was determined to be applicable.
      Use HyperDoc Browse, or issue
                                )display op D
      to learn more about the available operations. Perhaps
      package-calling the operation or using coercions on the arguments
      will allow you to apply the operation.

   Cannot find a definition or applicable library operation named D
      with argument type(s)
                               PositiveInteger
                                 Variable z

      Perhaps you should use "@" to indicate the required return type,
      or "$" to specify which version of the function you need.