public Main_Form()
{
InitializeComponent();
AddOwnedForm(init_form);
AddOwnedForm(extend_form);
AddOwnedForm(del_formula_form);
formula = new Formula();
label1.Text = "";
}
// Отмена последнего действия
private void button_cancel_last_active_Click(object sender, EventArgs e)
{
formula = formula.CancelPrevious();
textbox_current_formula.Text = formula.ToString();
}