Please help

[Thodosss s]

I have made a class that computes Dixon Elimination but something Null is printed , do you see the error ?

class DixonSub(Builtin):

     

    

    rules = {

        'DixonSub[expr_List,var_List,avar_List]': 'Block[{i,temp={expr}}, '

        'For[i=1,i<=Length[var],i++,AppendTo[temp,ReplaceAll[Last[temp],var[[i]]->avar[[i]]]]] '

        'If[Length[expr]===Length[var]+1 && Length[var]===Length[avar],MatrixForm[temp],"Error with the length of variables or auxiliary variables or polynomials"]]',

        

        'DixonSub[expr_List,var_List]': '"3 arguments are expected"',

        

        'DixonSub[expr_List]': '"3 arguments are expected"',

        

    }

and this is the output is the following

In[1]:= DixonSub[{x-1,x-2},{x},{a}]

Out[1]= Null -1 + x   -2 + x

        

        -1 + a   -2 + a