How can this assert() ever trigger?

[Albert van der Horst]

I have the following code for calculating the determinant of
a matrix. It works inasfar that it gives the same result as an
octave program on a same matrix.

/ ----------------------------------------------------------------

def determinant( mat ):
    ''' Return the determinant of the n by n matrix mat
        i row j column
        Destroys mat ! '''
    #print "getting determinat of", mat
    n=len(mat)
    nom = 1.
    if n == 1: return mat[0][0]
    lastr = mat.pop()
    jx=-1
    for j in xrange(n):
       if lastr[j]:
           jx=j
           break
    if jx==-1: return 0.
    result = lastr[jx]
    assert(result<>0.)
    # Make column jx zero by subtracting a multiple of the last row.
    for i in xrange(n-1):
        pivot = mat[i][jx]
        if 0. == pivot: continue
        assert(result<>0.)
        nom *= result   # Compenstate for multiplying a row.
        for j in xrange(n):
            mat[i][j] *= result
        for j in xrange(n):
            mat[i][j] -= pivot*lastr[j]
    # Remove colunm jx
    for i in xrange(n-1):
       x= mat[i].pop(jx)
       assert( x==0 )

    if (n-1+jx)%2<>0: result = -result
    det = determinant( mat )
    assert(nom<>0.)
    return result*det/nom

/-----------------------------------------

Now on some matrices the assert triggers, meaning that nom is zero.
How can that ever happen? mon start out as 1. and gets multiplied
with a number that is asserted to be not zero.

Any hints appreciated.

Groetjes Albert
-- 
Albert van der Horst, UTRECHT,THE NETHERLANDS
Economic growth -- being exponential -- ultimately falters.
albert at spe&ar&c.xs4all.nl &=n http://home.hccnet.nl/a.w.m.van.der.horst