docs: def _parametersAsIndex( self, ps ):
          """Private method to turn a parameter dict into a string suitable for
          keying a dict.
  
          ps: the parameters as a hash
          returns: a string key"""
          k = ""
          for p in sorted(ps.keys()):       # normalise the parameters
              v = ps[p]
              k = k + "{p}=[[{v}]];".format(p = p, v = v)
          return k