function change_formulier_sprint(i) {
  var contact_er1 = document.getElementById("contact_er1");
  var contact_er2 = document.getElementById("contact_er2");
  var contact_er3 = document.getElementById("contact_er3");
  var contact_er4 = document.getElementById("contact_er4");

  switch (i) {
    case "":   // Niets
      contact_er1.style.display = "none";
      contact_er2.style.display = "none";
      contact_er3.style.display = "none";
      contact_er4.style.display = "none";
      break;
    case "info":   // Meer info
      contact_er1.style.display = "";
      contact_er2.style.display = "";
      contact_er3.style.display = "none";
      contact_er4.style.display = "none";
      break;
    case "demo":    // Demo
      contact_er1.style.display = "";
      contact_er2.style.display = "";
      contact_er3.style.display = "";
      contact_er4.style.display = "";
      break;
    case "nieuws":    // Nieuwsbrief
      contact_er1.style.display = "none";
      contact_er2.style.display = "none";
      contact_er3.style.display = "none";
      contact_er4.style.display = "none";
      break;
    case "opendag":    // Open dag
      contact_er1.style.display = "none";
      contact_er2.style.display = "none";
      contact_er3.style.display = "none";
      contact_er4.style.display = "";
      break;
    }
}