docs: def strictly_monotonic(bb):
      '''
      bb is an index array which may have numerous double or triple
      occurrences of indices, such as for example the decay_index_pointer.
      This method removes all entries <= -, then all dublicates and
      finally returns a sorted list of indices.
  
      '''
      cc=bb[np.where(bb>=0)]
      cc.sort()
      dc=cc[1:]-cc[:-1] # subsequent equal entries have 0 in db
      dc=np.insert(dc,0,1) # the first element is always unique (the second occurence is the dublicate)
      dc_mask=np.ma.masked_equal(dc,0)
      return np.ma.array(cc,mask=dc_mask.mask).compressed()