% n sumints (sum(n))

/sumints { % straightforward recursive def
% note how the stack eventually has n+1 items on it
   dup
      0 eq
      {}
      {dup 1 sub sumints add}
   ifelse
} def


/sumints { % named parameter
% now the *dict* stack has n+1 dictionaries on it
   1 dict begin
      /i exch def
         i 0 eq
         {0}
         {i 1 sub sumints i add}
      ifelse
   end
} def

/sumints { % accumulating parameter
% uses constant stack space
   1 dict begin
      /sumints2 { % sumsofar ntogo sumints2 (sumsofar + (sum (ntogo))
         dup
            0 eq
            {pop} % 0 on stack from dup; answer is below it
            {  
               dup
               3 1 roll
               add
               exch
               1 sub
               sumints2
            }
         ifelse
      } def
      0 exch sumints2
   end
} def