def sphere(x0, y0, z0, r):
s = MathTree('-r++q%sq%sq%sf%g' % (('-Xf%g' % x0) if x0 else 'X',
('-Yf%g' % y0) if y0 else 'Y',
('-Zf%g' % z0) if z0 else 'Z',
r))
s.xmin, s.xmax = x0-r, x0+r
s.ymin, s.ymax = y0-r, y0+r
s.zmin, s.zmax = z0-r, z0+r
s.shape = True
return s
