stack_pop
exp_op = stack_pop(stk, exp1, NULL);
exp_op = stack_pop(stk, exp2, NULL);
exp1 = stack_pop(stk, exp_op, exp2);
exp_op = stack_pop(stk, exp3, NULL);
exp2 = stack_pop(stk, exp3, NULL);
exp_op = stack_pop(stk, exp2, exp3);
exp1 = stack_pop(stk, exp2, exp3);
exp = stack_pop(stk, NULL, NULL);
exp_op = stack_pop(stk, exp, NULL);
exp = stack_pop(stk, NULL, NULL);
while ((nexp = stack_pop(stk, NULL, NULL)) != NULL)