# 'sum' function

#define A p3
#define B p4
#define I p5
#define X p6

# leaves the sum at the top of the stack

Eval_sum = ->
  i = 0
  j = 0
  k = 0


  # 1st arg
  body = cadr(p1)

  # 2nd arg (index)
  indexVariable = caddr(p1)
  if (!issymbol(indexVariable))
    stop("sum: 2nd arg?")

  # 3rd arg (lower limit)
  push(cadddr(p1))
  Eval()
  j = pop_integer()
  if (isNaN(j))
    push p1
    return

  # 4th arg (upper limit)
  push(caddddr(p1))
  Eval()
  k = pop_integer()
  if (isNaN(k))
    push p1
    return


  # remember contents of the index
  # variable so we can put it back after the loop
  p4 = get_binding(indexVariable)

  push_integer(0)

  for i in [j..k]
    push_integer(i)
    p5 = pop()
    set_binding(indexVariable, p5)
    push(body)
    Eval()
    add()

  # put back the index variable to original content
  set_binding(indexVariable, p4)