code: def corr(sim=None, obs=None, node=None, skip_nan=False):
      
      sim, obs = prepare_arrays(sim, obs, node, skip_nan)
      return numpy.corrcoef(sim, obs)[0, 1]
docs: Calculate the product-moment correlation coefficient after Pearson.
  
      >>> from hydpy import round_
      >>> from hydpy import corr
      >>> round_(corr(sim=[0.5, 1.0, 1.5], obs=[1.0, 2.0, 3.0]))
      1.0
      >>> round_(corr(sim=[4.0, 2.0, 0.0], obs=[1.0, 2.0, 3.0]))
      -1.0
      >>> round_(corr(sim=[1.0, 2.0, 1.0], obs=[1.0, 2.0, 3.0]))
      0.0
  
      See the documentation on function |prepare_arrays| for some
      additional instructions for use of function |corr|.