# poly() gives polymonial, 
  poly(a,b,c,d) gives a*x^3+b*x^2+c*x+d,
  poly(a_ and b_) gives x^2-a*x-b*x+a*b, where a and b are roots of polymonial;


poly(a_,b_):=a*x+b;
poly(a_,b_,c_):=a*x^2+b*x+c;
poly(a_,b_,c_,d_):=a*x^3+b*x^2+c*x+d;
poly(a_,b_,c_,d_,f_):=a*x^4+b*x^3+c*x^2+d*x+f;

poly(a_ and b_) := x^2-a*x-b*x+a*b;
poly(a_ and b_ and c_) := expand((x-a)*(x-b)*(x-c));
poly(a_ and b_ and c_and d_) := expand((x-a)*(x-b)*(x-c)*(x-d));