function out = __foldfun(fun3, fun2, varargin);
  ## called by fold*.
  for ii=1:(nn-1);
    if ii==1;
      out = fun2(varargin{ii}, varargin{ii+1});
    else
      out = fun3(out, varargin{ii}, varargin{ii+1});
    endif
  endfor
endfunction