Fractional order transfer function in matlab

Hello all, I need an advice with this code:

function G=fotf(a,na,b,nb)

if nargin==0,

G.a=[]; G.na=[]; G.b=[]; G.nb=[]; G=class(G,'fotf');

elseif isa(a,'fotf'), G=a;

elseif nargin==1 & isa(a,'double'), G=fotf(1,0,a,0);

else,

ii=find(abs(a)<eps); a(ii)=[]; na(ii)=[];

ii=find(abs(b)<eps); b(ii)=[]; nb(ii)=[];

G.a=a; G.na=na; G.b=b; G.nb=nb; G=class(G,'fotf');

end

Syntax is : G=fotf(a,na,b,nb) its not my own this give an error :

??? Error using ==> class

The CLASS function must be called from a class constructor.

Error in ==> fotf at 9

G.a=a; G.na=na; G.b=b; G.nb=nb; G=class(G,'fotf');

Anyone know what is wrong will be a great help, thanks