[Xy-pic] how to increase the size of heads of arrows

[Bruno Sassatelli]

I have been using xy-pic for a while now, in papers, 
and i was very satisfied. It happened however this week that i had to
give a talk in which  i wanted to reuse some of the drawing i had made
using xy in the papers. It worked fine, with a major exception: 
the head of all the arrows is tiny, and almost invisible on the
screen. So people in the audience could see the lines of the arrows
i had, but could not see which direction the arrow was going (i am
using simple arrows of the form ->). 
Can somebody tell me how to make the head larger? 
I need it urgently, as i have to give a talk tomorrow.
Thanks,
Bruno
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