docs: def onepara(R):
      """Converts an ill-conditioned correlation matrix
         into well-conditioned matrix with one common
         correlation coefficient
  
      Parameters:
      -----------
      R : ndarray
          an illconditioned correlation matrix,
          e.g. oxyba.illcond_corrmat
  
      Return:
      -------
      cmat : ndarray
          DxD matrix with +1 as diagonal elements
          and 1 common coefficient for all other
          relations.
      """
      import numpy as np
      import warnings
  
      d = R.shape[0]
  
      if d < 2:
          raise Exception((
              "More than one variable is required."
              "Supply at least a 2x2 matrix."))
  
      # the explicit solution
      x = (np.sum(R) + np.trace(R)) / (d**2 - d)
  
      if x < (-1. / (d - 1)) or x > 1:
          warnings.warn("No analytic solution found x={:.8f}".format(x))
          return None
      else:
          C = np.eye(d)
          C[np.logical_not(C)] = x
          return C