function a=plistr(n,in,out);
   for i=1:n;
      a(i)=fscanf(in,'%lf',1); 
      if (length(a(i)) == 0);
         fprintf(out,'error in reading list of positive constants.\n');
         fprintf(out,'program stops');
         quit;
      end;
      if (a(i) <= 0);
         fprintf(out,'*** element %3d of list= %15.7e is not positive\n',i,a(i));
         fprintf(out,'program stops');
         quit;
      end;
   end;                          
end;