+ * One extra round of reduction, for cc*2^256, which means
+ * adding cc*(2^224-2^192-2^96+1) to a 256-bit (nonnegative)
+ * value. If cc is negative, then it may happen (rarely, but
+ * not neglectibly so) that the result would be negative. In
+ * order to avoid that, if cc is negative, then we add the
+ * modulus once. Note that if cc is negative, then propagating
+ * that carry must yield a value lower than the modulus, so
+ * adding the modulus once will keep the final result under
+ * twice the modulus.