%%HP: T(3)A(R)F(.);
\<< \-> sumidx low high
summand
  \<< sumidx low \->LST
TAIL REVLIST TAIL
REVLIST \->PRG high
\->LST TAIL REVLIST
TAIL REVLIST \->PRG
summand \->LST TAIL
REVLIST TAIL REVLIST
\->PRG { \GS } HEAD 5
\->ALG
  \>>
\>>