Xy-pic command doesn't seem to work

[Alberto Alcalá Alvarez]

\xymatrix {     B \ar[r] &R}
types out:       B  ---> R  
while
\xymatrix {     B \ar {-->} [r] & R}does something like       B>^[r]     RI 
already tried several combinations for spaces and brackets and even tried other 
arrow modifiers, checked the XY-guide and the XY tutorial, yet I can't get 
things like   B - - - >R or B  >- - - >R. I keep getting error messages:  
"command used out of context", "extra { or forgotten $", "missing { inserted". 
by the way, i'm using mac osX and lyx 2.0.. hope somebody can help me out, 
thanks