Lignum vitae wood can also be used to brew a tea. Other names for lignum vitae include palo santo (Spanish for "holy wood") Aura palo santo and "bastard greenheart" (not to be confused with true greenheart "Chlorocardium rodiei", a popular wood in shipbuilding, cabinetry, and woodturning but a completely different timber). Lignum vitae is also one of the numerous hard, dense woods loosely referred to as "ironwood". Lignum vitae is hard and durable, and is also the densest wood traded (density: 1.23 g/cm); it will easily sink in water. On the Janka Scale of Hardness, which measures hardness of woods, lignum vitae
NP-hardness nor "Undecidable". For instance, the language of True quantified Boolean formulas is decidable in polynomial space, but not in non-deterministic polynomial time (unless NP = PSPACE). NP-hard problems do not have to be elements of the complexity class NP. As NP plays a central role in computational complexity, it is used as the basis of several classes: NP-hard problems are often tackled with rules-based languages in areas including: NP-hardness NP-hardness (non-deterministic polynomial-time hardness), in computational complexity theory, is the defining property of a class of problems that are, informally, "at least as hard as the hardest problems in NP". A