[R] details of cor function

[David Parkhurst]

How can I find out whether the cor function with method="Kendall" 
computes Kendall's tau-a or tau-b.  I understand that tau-b deals better 
with ties, and I'm wanting to look for correlation in two variables that 
have lots of ties (especially lots of zeroes for one of them).  The 
information provided by ?cor does not specify which is computed.
And a question: am I better off to jitter the variables before computing 
tau, given the many ties?