function nr=xberni(X, r) 
% xberni(X, r)  Generates a column vector for the wait time for the rth 
%    positive in each expirement of the Bernoulli random variable matrix X. 
%    If the rth positive does not occur, the result is infinity.  The  
%    resulting vector is a Pascal random variable. 
if sum(sum(X==1))+sum(sum(X==0))<prod(size(X)) 
   error('not a Bernoulli random variable') 
end 
if r<=0 
   error('invalid value for r') 
end 
s=size(X); 
nr=zeros(1, s(1)); 
for i=1:s(1), 
   f=find(X(i,:)==1); 
   if r>length(f) 
      nr(i)=Inf; 
   else 
      nr(i)=f(r); 
   end 
end