linear( a, ka, b, kb )
	mp a, b;
	int ka, kb;

	/*** compute a*ka + b*kb --> a ***/
	{int i, la, lb;
	la = length( a );	lb = length( b );
	for (i=1; i<la; i++) a[i] *= ka;
	if ( sign(a)!=sign(b) )	kb = -kb;
	if ( lb>la ) {
		storelength( a, lb );
		for (i=la; i<lb; i++) a[i] = 0;
		}
	for (i=1; i<lb; i++) a[i] += kb*b[i];
	normalize( a );
	};