pub const NDF: usize = 3;Expand description
Degrees of freedom per node: 2D translation (x, y) plus in-plane
rotation (z). The third DOF carries ElasticBeamColumn’s
bending. A node’s DOF becomes an equation only if something uses it
(an element stiffens it, a constraint ties to it, it carries a mass, or
it is fixed), so a node connected only to trusses has no rotation
unknown and needs none fixed by hand (see Domain::is_active).