[Xy-pic] Change the orientation of Arrow of xtwocell

[Michael Barr]

I don't know in native xy-pic, but if you use diagxy (a front end to 
xy-pic), it is pretty easy:
  $$\bfig
  \ptriangle/>`>`<--/<800,500>[A`C`B;X`F`R]
  \morphism(50,200)/=>/<200,0>[`;\eta]
  \efig$$
seems to do just what you wanted.  Diagxy (including a tutorial) is 
avaiable from CTAN.

Michael Barr

-- 
The modern conservative is engaged in one of man's oldest exercises in
moral philosophy--the search for a superior moral justification
for selfishness.  --J.K. Galbraith