allpass filter for parametric equalizer | FX Book Questions | Forum

Hallo everyone
thank you for accepting me as member of this forum i have a code which implements a 1 order lowshelving filter using a 1 order allpass filter
but i have some questions can i make some questions the code is thisone some parts of this code are from a book, some parts are my
Now can i ask some questions????????????????

function [ Hd] = lowshelving( x,Wc,G )
V0=10^(G/20);
H0=V0-1;
H01=H0/2;
if G >=0
c=(tan(pi*Wc/2)-1)/(tan(pi*Wc/2)+1); %b
else
c=(tan(pi*Wc/2)-V0)/(tan(pi*Wc/2)+V0); %c
end;
xh=0;
for n=1:length(x)
xh_new=x(n)-c*xh;
ap_y=c*xh_new+xh;
xh=xh_new;
y(n)=0.5*H0*(x(n)+ap_y)+x(n);
end;
b0=1+(1+c)*H01
b1=c+(1+c)*H01
a0=1
a1=c

b = [ b0,b1]
a = [ a0,a1]
[sos,g]=tf2sos(b,a);
Hd=dfilt.df2sos(sos,g);