code: public Matrix4 setToSkew (double a, double b, double c, double d, double x, double y, double z) {
          return set(1f + a*x, b*x, c*x, d*x,
                     a*y, 1f + b*y, c*y, d*y,
                     a*z, b*z, 1f + c*z, d*z,
                     0f, 0f, 0f, 1f);
      }
docs: Sets this to a skew by the specified amount relative to the given plane.
  
  @return a reference to this matrix, for chaining.