=begin
  # sample-groebner03.rb

  require "algebra"
  F5 = ResidueClassRing(Integer, 2)
  F = AlgebraicExtensionField(F5, "a") {|a| a**3 + a + 1}
  a = F.var
  P = MPolynomial(F)
  
  x, y, z = P.vars("xyz")
  f1 = x + y**2 + z**2 - 1
  f2 = x**2 + z**2 - y * a
  f3 = x - z - a
  
  f = x**3 + y**3 + z**3
  q, r = f.divmod_s(f1, f2, f3)
  p f == q.inner_product([f1, f2, f3]) + r #=> true
((<_|CONTENTS>))
=end