A remark on "an approximation condition and extremal quasiconformal extension

LET D be the unit disk in the complex plane C and f be a sense preserving quasisymmetrichomeomorphism of D onto itself. Denote by Q a quadrilateral D （z1, z2, z3, z4 ） with do-main D and vertices z1, z2, z3, z4 ∈D, and by M（Q） its conformal modulus. We are in-