  A simple ansatz is proposed for two-color R-matrix satisfying the tetrahedron
equation. It generalizes, on one hand, a particular case of the eight-vertex
model to three dimensions, and on another hand - Hietarinta's permutation-type
operators to their linear combinations. Each separate R-matrix depends on one
parameter, and the tetrahedron equation holds provided the quadruple of
parameters belongs to an algebraic set containing five irreducible
two-dimensional components.
