The routine returns true if the plane p is tangent to the sphere s; false if they are not; and FAIL if it is unable to reach a conclusion.

In case of FAIL, if the third optional argument is given, the condition that makes p tangent to s is assigned to this argument.

The command with(geom3d,IsTangent) allows the use of the abbreviated form of this command.

IsTangent:   "unable to determine if 1-abs(c+d)/(a^2+b^2+c^2)^(1/2) is zero"