def _parametersAsIndex( self, ps ):
k = "<STR_LIT>"<EOL>for p in sorted(ps.keys()):       <EOL><INDENT>v = ps[p]<EOL>k = k + "<STR_LIT>".format(p = p, v = v)<EOL><DEDENT>return k<EOL>
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