+ If we get a zero remainder, then the numerator should belong to
+ the ideal generated by the denominators::
+
+ sage: set_random_seed()
+ sage: R = PolynomialRing(QQ, 'x,y,z')
+ sage: x,y,z = R.gens()
+ sage: s = ZZ.random_element(1,5).abs()
+ sage: gs = [ R.random_element() for idx in range(s) ]
+ sage: f = R.random_element(ZZ.random_element(10).abs())
+ sage: (qs, r) = multidiv(f,gs)
+ sage: r != 0 or f in R.ideal(gs)
+ True
+
+ The numerator is always the sum of the remainder and the quotients
+ times the denominators, and the remainder's monomials aren't divisible
+ by the leading term of any denominator::
+
+ sage: set_random_seed()
+ sage: R = PolynomialRing(QQ, 'x,y,z')
+ sage: x,y,z = R.gens()
+ sage: s = ZZ.random_element(1,5).abs()
+ sage: gs = [ R.random_element() for idx in range(s) ]
+ sage: f = R.random_element(ZZ.random_element(10).abs())
+ sage: (qs, r) = multidiv(f,gs)
+ sage: r + sum( qs[i]*gs[i] for i in range(len(gs)) ) == f
+ True
+ sage: r == 0 or (not any( g.lt().divides(m) for m in r.monomials()
+ ....: for g in gs ))
+ True