  Scattering of a plane electromagnetic wave by an anisotropic impedance
right-angled concave wedge at skew incidence is analyzed. A closed-form
solution is derived by reducing the problem to a symmetric order-2 vector
Riemann-Hilbert problem (RHP) on the real axis. The problem of matrix
factorization leads to a scalar RHP on a genus-3 Riemann surface. Its solution
is derived by the Weierstrass integrals. Due to a special symmetry of the
problem the associated Jacobi inversion problem is solved in terms of elliptic
integrals, not a genus-3 Riemann \`e-function. The electric and magnetic field
components are expressed through the Sommerfeld integrals, and the incident and
reflected waves are recovered.
