anchor.diff:      !     \frac{\partial H_{N-1}}{\partial \eta} & \frac{\partial H_{N}}{\partial \eta} \end{array}
       ! \right]\]
       !
  -    ! \[ X = \left[ \begin{array}{c}
  +    ! \[ \boldsymbol{X} = \left[ \begin{array}{c}
       !      \begin{array}{cc}
       !        x_1 & y_1 \\
       !        x_2 & y_2
  
positive.code:     ! Calculates the Jacobian of a quadrilateral element
      !
      ! The Jacobian of an element is defined as:
      ! \[ \textbf{J} = \textbf{P} \textbf{X} \]
      !
      ! Where:
      ! \[ \textbf{P} = \left[ \begin{array}{cc}
      !     \frac{\partial H_1}{\partial \xi} & \frac{\partial H_2}{\partial \xi} \\
      !     \frac{\partial H_1}{\partial \eta} & \frac{\partial H_2}{\partial \eta} \end{array}
      !       \cdots
  
negative.code:     !      \end{array} \\
      !    \end{array} \right]\]
      !
      ! Thus:
      !
      ! \[ \boldsymbol{J} =  \left[ \begin{array}{cc}
      !           \boldsymbol{H}_{\xi} \cdot \boldsymbol{x} & \boldsymbol{H}_{\xi} \cdot \boldsymbol{y} \\
      !           \boldsymbol{H}_{\eta} \cdot \boldsymbol{x} & \boldsymbol{H}_{\eta} \cdot \boldsymbol{y} \\
      !         \end{array} \right] \]
      !
      !
      ! * \( \boldsymbol{H} \) : Vector of basis functions
      ! * \( \boldsymbol{x} \) : Vector of X-coordinates for all element nodes
      ! * \( \boldsymbol{y} \) : Vector of Y-coordinates for all element nodes
  
      ! * \( H_i \) : Basis function \(i\)
      ! * \( x_i \) : X-coordinate of node \(i\)
      ! * \( y_i \) : Y-coordinate of node \(i\)
  