def log_ncr(a, b):
val = gammaln(a+<NUM_LIT:1>) - gammaln(a-b+<NUM_LIT:1>) - gammaln(b+<NUM_LIT:1>)<EOL>return val<EOL>
Returns log(nCr(a,b)), given that b<a. Does not assume that a and b
are integers (uses log-gamma).