[Xy-pic] problem with bending arrows

[Thierry Masson]

Hello,

I am trying to understand bending arrows, and the documentations I  
find here and there is not clear enough for me! If someone could give  
me a very pedestrian introduction, with lot of illustrated exemples,  
it would help.
Here is my question.
I have succeed to produce the following diagram:

$
\xymatrix at R=15pt at C=25pt{
{} & {A_0} \ar[d] \ar[r] & {B_0} \ar[d] \ar[r] & {C_0} \ar[d]
\ar@<-0.5pc>`d[dl]+/u0.3pc/ % turn at C1
`[ddl]+/l0.3pc/ % turn at B1
`[ddll]+/u0.3pc/+/l1pc/ % turn at B2
`[dddll]+/l0.2pc/% turn at A2
_-{\partial}%
[dddll]+/l0.2pc/+/u0.5pc/ %
& {} \\
{} & {A_1} \ar[d] \ar[r] & {B_1} \ar[d] \ar[r] & {C_1} \ar[d] \ar[r]  
& {0} \\
{0} \ar[r] & {A_2} \ar[d] \ar[r]  & {B_2} \ar[d] \ar[r] & {C_2} \ar 
[d] & {} \\
{} & {A_3} \ar[r] & {B_3} \ar[r] & {C_3}& {}
}
$

I can't understand why the turns at B1 and A2 don't look like the  
turns at C1 and B2. I prefer the C1 and B2 turns. How to modify my  
diagram to get the same radius at B1 and A2?
I can't understand why the last straight line from A2 to A3 is not  
perfectly vertical... What is wrong about my coding?

Thanks for any advise,
Thierry