code: def conductivity_prior_mean_3d_mf(z, x):
      """Prior mean for the conductivity with multi-fidelity."""
      normalised_z = (z[0] - 60.0) / 120.0
      vol1 = x[0]  # LiNO3
      vol2 = x[1]  # Li2SO4
      vol3 = x[2]  # NaClO4
      vol4 = 10 - (vol1 + vol2 + vol3)  # Water
      # Synthetic functions
      return (
          0.9 * vol1 * normalised_z
          + 0.12 * (vol2 + vol3) ** 2.005 * (normalised_z**1.55)
          + 2.4 * vol4 * (vol1**1.5)
      )
  
label_desc: Use image node_id 2 for calling a global function with example usage: conductivity_prior_mean_3d_mf(z, x) and returns: unknown