[Maxima] Queries about use of maxima

[John Ogilvie]

A further query arises in connexion with some material that I am preparing
for demonstration to school teachers for their use of Maxima for teaching
mathematics (in Spanish).
      When I input into wxMaxima 0.8.3 (or xMaxima 5.19.2) this command,
     plot2d([(1-x^2)^(1/2),-(1-x^2)^(1/2)],[x,-1.9,1.9],[y,-1.1,1.1])
the expected plot appears, and with the indicated domain of x the circle
even resembles a circle.  However, when I input the analogous command for
plot3d, extending (1-x^2)^(1/2) to (1-x^2-y^2)^(1/2) in both positive and 
negative parts (to obtain the two hemispheres), an error message
      List [sqrt(-y^2-x^2+1),-sqrt(-y^2-x^2+1)] is not of length 3
appears, which is incomprehensible.
      Yes, I am aware that for the case of a circle, in just two 
dimensions, I can instead use implicit_plot, but there seems to be 
available no equivalent in Maxima for three dimensions.
      Does that error message arise from my error or Maxima's?
      J. F. Ogilvie