"that classes of over 20 students are unconstitutional" (295 2 at 11), set forth no acceptable basis to disturb the trial court's finding.[5] We conclude that plaintiffs' evidence of the advantages of smaller class sizes supports the inference sufficiently to show a meaningful correlation between the large classes in City schools and the outputs to which we soon turn. In sum, the Appellate Division erred in concluding that there was not "sufficient proof" (295 2 at 11) that large class sizes negatively affect student performance in New York City public schools. Instrumentalities of Learning. The final input is "instrumentalities of learning," including classroom supplies, textbooks, libraries and computers. The courts below agreed that the textbook supply is presently adequate and the evidence on classroom supplies is inconclusive. On the other hand, evidence including the latest 655 Report showed that New York City schools had about nine library books per student -- half as many as schools statewide excluding the City, and just under half the number recommended by the American Library Association. In light of Levittown, the intrastate inequality does not prove anything in itself, and a library association might be expected to advocate book purchases at levels exceeding the constitutional floor. But in holding that the library books in New York City schools are "inadequate in number and quality" (187 Misc 2d at 57) the trial court clearly relied on the abundant testimony on the adequacy of the books for pedagogical purposes rather than on purely numerical intrastate comparisons. The unrebutted testimony indicated that the books in City school libraries are old and not integrated with contemporary curricula. The Appellate Division suggested that school libraries simply consist of "classics" rather than "multicultural" books (295 2 at 12), but the record contains not one scintilla of evidence that antiquated books in City school libraries are "classics." The Appellate Division thus gave no factual basis for its disagreement with the trial court that the library books in New York City schools are inadequate in quality. The record concerning computers is similar, establishing that some exposure to them has become essential and that City schools not only have about half as many computers per student as all other New York schools, but also have aging equipment that, in some cases, simply cannot support presently- available software. The Appellate Division speculated that old equipment might be used "for introductory classes" (295 2 at 11), but this possibility was not even advocated by the State and, like the "classic" outdated library books, has no record support at all. While we hesitate to overstate the importance of libraries and