Optional
At most one entry per row (core::ZeroLength allows only one friction).
core::ZeroLength
Sparse (zero_length row index, dof, material arena index) — most DOFs on a given ZeroLength carry no material at all.
(zero_length row index, dof, material arena index)
ZeroLength
At most one entry per row (
core::ZeroLengthallows only one friction).