function X=xval(cf, P) 
% xval(cf, P)  gives the x value of the given cumulative function at the 
%              given value of P.  F() inverse. 
if iscf(cf)==0 
   error('invalid cumulative function') 
end 
if min(P)<0 | max(P)>1 
   error('invalid P value') 
end 
P=P(:)'; 
X=zeros(size(P)); 
x=xvals(cf); 
p=pvals(cf); 
if isdiscrt(cf)==0 
   for i=1:length(P), 
      j=round(P(i)*length(x)+0.5); 
      if j>length(x) 
         j=length(x); 
      end 
      if P(i)>=p(j) 
         while j<length(p) & P(i)>=p(j) 
            if j<=length(p)-10 
               if P(i)>p(j+10) 
                  j=j+10; 
               else 
                  j=j+1; 
               end 
            else 
               j=j+1; 
            end 
         end 
         X(i)=x(j); 
      else 
         j=j-1; 
         while j>0 & P(i)<p(j+1) 
            if j>10  
               if P(i)<p(j-9) 
                  j=j-10; 
               else 
                  j=j-1; 
               end 
            else 
               j=j-1; 
            end 
         end 
         if j<length(x)-1 & j>0 
            j=j+1; 
         elseif j==0 
            if P(i)>p(1) 
               j=1; 
            else 
               j=0; 
            end 
         end 
         X(i)=x(j+1); 
      end 
   end 
else 
   for i=1:length(P), 
      index=find(p>=P(i)); 
      X(i)=x(index(1)); 
   end 
end