Function: trace
Section: linear_algebra
C-Name: gtrace
Prototype: G
Help: trace(x): trace of x.
Doc: this applies to quite general $x$. If $x$ is not a
 matrix, it is equal to the sum of $x$ and its conjugate, except for polmods
 where it is the trace as an algebraic number.

 For $x$ a square matrix, it is the ordinary trace. If $x$ is a
 non-square matrix (but not a vector), an error occurs.
