The XMULT function computes a matrix product by using extended-precision calculations. For most matrices, the the XMULT function is numerically equivalent to the matrix multiplication operator (*). You should use the XMULT function on pathalogical examples for which extended-precision calculations are required to obtain an accurate product.
The following program demonstrates the use of the XMULT function:
a = 1e13; b = 1e13; c = 100*a; a = a+1; x = c || a || b || c; y = c || a || (-b) || (-c); z = xmult(x,y`); /* correct answer */ print z [format=16.0]; wrong = x * y`; /* loss of precision */ print wrong [format=16.0];