###
 Return the leading coefficient of a polynomial.

Example

  leading(5x^2+x+1,x)

Result

  5

The result is undefined if P is not a polynomial.
###

Eval_leading = ->
  push(cadr(p1))
  Eval()
  push(caddr(p1))
  Eval()
  p1 = pop()
  if (p1 == symbol(NIL))
    guess()
  else
    push(p1)
  leading()

#define P p1
#define X p2
#define N p3

leading = ->
  save()

  p2 = pop()
  p1 = pop()

  push(p1)  # N = degree of P
  push(p2)
  degree()
  p3 = pop()

  push(p1)  # divide through by X ^ N
  push(p2)
  push(p3)
  power()
  divide()

  push(p2)  # remove terms that depend on X
  filter()

  restore()