= Constraint reaction in hyperelastic stress
{title2=$N_{\alpha i}=W_{,A_{i\alpha}}+qF_{,A_{i\alpha}}$}
For a regular material constraint $F(A)=0$, admissible rates of the <deformation gradient> lie in the kernel of $F_{,A}$. The difference between the <nominal stress> and the derivative of the <strain energy density> annihilates that tangent space by <virtual work>, so it is a <Lagrange multiplier> times $F_{,A}$. For <incompressibility>, the derivative of the <determinant> is its <cofactor matrix>, and the material-first reaction is $qA^{-1}$ when $\det A=1$.
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