sequence combinations = {}

procedure recurse(sequence remaining,               /* digits remaining to compute */
                  integer ne,                       /* number of digits expected for a valid combination */
                  sequence selected = {})           /* digits already selected for a potential combination */
  integer nr = length(remaining)   -- number of digits remaining to compute
  integer ns = length(selected)    -- number of digits already selected for a potential combination
  if ns+nr = ne then
    combinations = append(combinations, selected & remaining)
    return
  end if
  if nr = 1 or ns = ne then
    combinations = append(combinations, selected)
    return
  end if
  -- select next
  recurse(remove(remaining, 1), ne, selected & remaining[1])
  -- skip next
  recurse(remove(remaining, 1), ne, selected)
end procedure

function get_combinations(sequence s, integer lg)
  combinations = {}
  recurse(s, lg)
  return combinations
end function

constant TEST_SEQUENCE = {0,1,2,3,4}

combinations = {}
recurse(TEST_SEQUENCE, 3)
? combinations