function p = polynom(a)
%POLYNOM多項式類構造函數
%   p = POLYNOM(v) 創建creates a polynomial object from the vector v,
%   containing the coefficients of descending powers of x.
if nargin == 0
   p.c = [];
   p = class(p,'polynom');
elseif isa(a,'polynom')
   p = a;
else
   p.c = a(:).';
   p = class(p,'polynom');
end