Renamed cluster_env to cluster_access, moved private parts of env_t together, put some comments in.
Made set_func take dummy_var_t as input, return a sym_t, to ensure that people do not just go and wildly use sym_t values in a scope they have not created.
Made new_pluck_func and new_eq_comparison_func use the new safer pb::set_func instead of manually constructing a FUNC.