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

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