void convolve ( double *a , int *na , double *b ,
  int *nb , double *ab )
{
    int i , j , nab = *na + *nb - 1;

    for ( i = 0; i < nab ; i ++)
      ab [ i ] = 0.0 ;
    for ( i = 0; i < *na ; i ++)
        for ( j = 0; j < *nb ; j ++)
          ab [ i + j ] += a [ i ] * b [ j ] ;
 }