The 1991 Constitution sets up a relatively flexible system of constitutional change, which can be activated through several different routes. The Constitution has thus been frequently amended since its enactment. The Colombian Constitutional Court has been aggressive in policing the tools of constitutional change. It has been meticulous in ensuring that amendments follow the proper congressional procedures, and in the case of amendments via referendum it has ensured that questions are sufficiently clear and do not bias voters toward certain answers. Most significantly, the Court had developed a substitution of the constitution doctrine, where it has struck down some constitutional amendments on the ground that they replaced core provisions of the existing constitution, rather than merely changing them. This doctrine was most famously used in a case striking down a constitutional amendment that would have allowed the popular president Alvaro Uribe to run for a third straight term.