#5212 trigreduce rewrites asymmetrically

[David Scherfgen]

/* no rewrite: */

(%i1) trigreduce(1 - tanh(x)^2);
(%o1) 1-tanh(x)^2


/* here it does: */

(%i2) trigreduce(y*coth(x)^2 - y);
(%o2) csch(x)^2*y

(%i3) trigreduce(1/(1+tan(x)^2));
(%o3) 1/sec(x)^2

(%i4) trigreduce(1/sec(x)^2);
(%o4) (cos(2*x)+1)/2