spacing objects and arrows in xymatrix diagrams

[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 de tail of the 
arrow, or if I could rescale the shaft of the arrow in order to make it 
shorter. thanks a lot!