code: public static double dot( DMatrix2 a , DMatrix2 b ) {
          return a.a1*b.a1 + a.a2*b.a2;
      }
docs: <p>Performs the vector dot product:<br>
  <br>
  c = a * b <br>
  <br>
  c &ge; &sum;<sub>k=1:n</sub> { b<sub>k</sub> * a<sub>k</sub> }
  </p>
  
  @param a The left vector in the multiplication operation. Not modified.
  @param b The right matrix in the multiplication operation. Not modified.
  @return The dot product