[Xy-pic] spacing objects and arrows using xymatrix

[Alberto Alcalá Alvarez]

hello, I'm trying to get the
 following done, using the xymatrix command: I have a diagram with a >>--->> arrow going down, but the tail of the arrow, >>, gets overlapped with the name of the origin of the arrow, so my question is if there is a way to save some space between the name of the origin and the tail of the arrow, or if I could rescale the shaft of the arrow in order to make it shorter. thanks a lot!
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