  According to the Schwarz symmetry principle, every harmonic function
vanishing on a real analytic curve has an odd continuation, while a harmonic
function satisfying homogeneous Neumann condition has the even continuation.
There are different generalizations of the Schwarz symmetry principle. Most of
them are dealing with homogeneous conditions and the Dirichlet case. Using a
technique of Dirichlet to Neumann and Robin to Neumann operators, we derive
reflection formulae for nonhomogeneous Neumann and Robin conditions from a
reflection formula subject to a nonhomogeneous Dirichlet condition.
