log.sum <- function(v) {
   log.sum.pair <- function(x,y)
     {
       if ((y == -Inf) && (x == -Inf))
         { return(-Inf); }
       if (y < x) return(x+log1p(exp(y-x)))
       else return(y+log1p(exp(x-y)));
     }

   r <- v[1];
   for (i in 2:length(v))
     r <- log.sum.pair(r, v[i]);
   return(r);
}