xhash(convfunc = function(state, ...) state, convfunc_vec = function(states,
...) unlist(Map(convfunc, states, ...)), default_value = NULL)state and keyword arguments
represented by ..., and returns a character.states
and keyword arguments ...
and returns character vector. By default, it tries to vectorize
convfunc using Map. User may specify more efficient function
if any.xhash object