error in plot command?



x[1]:.5; x[2]:sqrt(1-x[1]^2) * cos(2*tau); x[3]:sqrt(1-x[1]^2) * sin 
(2*tau);
plot2d([x[2],x[3]], [tau,0,2*%pi]);
gives a plot of the cosine & sine over two periods, as expected

but when I try to plot the circle, parametrized by tau with:
plot2d([parametric, x[2],x[3], [tau,0,2*%pi]]);
maxima crashes with:
*** - argument to COMMON-LISP:FLOAT should be a real number: ((MTIMES  
SIMP) 0.8660254037844386 ((%COS SIMP) ((MTIMES SIMP) 2 $TAU)))
The following restarts are available:
R1 = Macsyma top-level

1. Break [1]>

Anyone tell me what I'm doing wrong?
And how do you exit one of those break [1]> things?  I have to close  
xmaxima and start again.

thanks
dan