code: public Matrix4x3d m12(double m12) {
          this.m12 = m12;
          properties &= ~PROPERTY_ORTHONORMAL;
          if (m12 != 0.0)
              properties &= ~(PROPERTY_IDENTITY | PROPERTY_TRANSLATION);
          return this;
      }
docs: Set the value of the matrix element at column 1 and row 2.
  
  @param m12
  the new value
  @return this