[Xy-pic] xy-pic arrow command doesn't work in LYX

[alberto]

hello, I've been trying for several days now to get different arrow styles in 
commutative diagrams, using LYX. I already read de xyguide, the manual, 
the reference and some other documentation, 
yet can't get it done. for example, for the following code:    
\xymatrix{R\ar@{-->}[r] B}       

I get the error message: 
Xy-pic error: illegal <arrow>: macro ->> not valid..... 
see the xypic manual for further info... 

but I went over the 
manual several times, and nothing seems to work. 
hope you can help me out. thanks!