inversion of a symmetric matrix in place.  algorithm 150, comm.
acm, h. rutishauser.
a is half of a symmetric matrix, n by n, arranged shortest rows
first.  that is, the first row is 1 long, the next 2, ... the
last n.  element a(i,j) (i.ge.j) is at a(j+i*(i-1)/2).
b is constant array of dimension n by m.  the solution of the
matrix equation ax=b is left in b.
pvrow, pvcol and iuse are temporary arrays of length n.  pvrwb
is a temporary array of length m.