This is a bug in the __call__ method of multivariate polynomial rings,
which doesn't respect user-defined coercions. The offending lines are
805-813 in sage/rings/polynomial/multi_polynomial_libsingular.pyx:
if isinstance(element, MPolynomial_libsingular) and
element.parent() is not self
I think that the following behavior is wrong.
sage: K=QQ['t,s']
sage: L=QQ['t0,t1,s0,s1']
sage: L.inject_variables()
Defining t0, t1, s0, s1
sage: Hom(K,L)([t0+t1,s0]).register_as_coercion()
sage: L.coerce_map_from(K)
Ring morphism:
From: Multivariate Polynomial Ring in t, s over Rational F
