Symbolic derivitives of 'trunc(x)', and 'ceil(x)

Does anyone know what this means?? When I take a symbolic derivitive of trunc(x) with respect to x I get trunc(1,x). This function can not be evaluated - to many arguments!

And another one:

When I take the symbolic derivitive of ceil(x) with respect to x, I get floor(1,x). As before, this function can not be evaluated.

Anyone know what it means when a built in function behaves in this way?

Dan