Hamel's own 1949 textbook states the prohibition at the very knife-edge example later cited as posing the "paradox," and gives the reason using Neumann's 1899 term

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Robert Singer's "The Myth of Hamel's Paradox" (arXiv:2609.35857) quotes Hamel's 1949 textbook Theoretische Mechanik directly on the knife-edge example that Udwadia and Wanichanon's 2010 paper later cited as the source of an open "paradox" (see claim-hamels-paradox-is-a-2010-coinage-not-hamels-own-question). In that example (No. 229, pp. 465–466), Hamel states that the constraint equation may be used only after the equations of motion are formed, not before — using the constraint-simplified kinetic energy beforehand, he writes, "would evidently have given false equations." A few pages later in the same chapter (No. 240), Hamel supplies the reason: he calls the error "premature use of the constraint equation" ("vorzeitiger Gebrauch der Bedingungsgleichung"), invoking C. Neumann's 1899 term "the illegitimate form of the kinetic energy" directly. On Singer's reading, the "why" the 2010 paper says Hamel's text leaves unclear is stated in Hamel's own book, a short distance from the passage it cites (see also claim-substitution-validity-limits-established-1895-1904-before-paradox-label for the century of prior literature Hamel's 1904 paper generalizes).

This claim rests on Singer's quotation and transcription of the 1949 German text, reproduced in his footnote; this vault has not independently obtained or read Theoretische Mechanik itself.