rem: dm_env = scheme_environment_from_dummy(dummy);
add: if (!dm_env) dm_env = scheme_environment_from_dummy(dummy);
context: do_define_syntaxes_execute(Scheme_Object *form, Scheme_Env *dm_env, int for_stx){  Scheme_Thread *p = scheme_current_thread;  Resolve_Prefix *rp;  Scheme_Object *base_stack_depth, *dummy;  int depth;  Scheme_Comp_Env *rhs_env;  rp = (Resolve_Prefix *)SCHEME_CAR(form);  base_stack_depth = SCHEME_CADR(form);  depth = SCHEME_INT_VAL(base_stack_depth) + rp->num_stxes + 1;  if (!scheme_check_runstack(depth)) {    p->ku.k.p1 = form;    if (!dm_env) {      /* Need to get env before we enlarge the runstack: */      form = SCHEME_CDDR(form);      dummy = SCHEME_CAR(form);      dm_env = scheme_environment_from_dummy(dummy);    }    p->ku.k.p2 = (Scheme_Object *)dm_env;    p->ku.k.i1 = for_stx;    return (Scheme_Object *)scheme_enlarge_runstack(depth, define_syntaxes_execute_k);  }  form = SCHEME_CDDR(form);  dummy = SCHEME_CAR(form);  form = SCHEME_CDR(form);  rhs_env = scheme_new_comp_env(scheme_get_env(NULL), NULL, SCHEME_TOPLEVEL_FRAME);  dm_env = scheme_environment_from_dummy(dummy);  scheme_on_next_top(rhs_env, NULL, scheme_false, NULL, dm_env, dm_env->link_midx);  return define_execute(SCHEME_CAR(form), SCHEME_CDR(form), for_stx ? 2 : 1, rp, dm_env);}