docs: def SLIT_RECTANGULAR(x,g):
      """
      Instrumental (slit) function.
      B(x) = 1/γ , if |x| ≤ γ/2 & B(x) = 0, if |x| > γ/2,
      where γ is a slit width or the instrumental resolution.
      """
      index_inner = abs(x) <= g/2
      index_outer = ~index_inner
      y = zeros(len(x))
      y[index_inner] = 1/g
      y[index_outer] = 0
      return y