attachPseudoElement(element, previousElement, hydrogenCount = 0, charge = 0) {
    if (hydrogenCount === null) {
      hydrogenCount = 0;
    }

    if (charge === null) {
      charge = 0;
    }

    let key = hydrogenCount + element + charge;

    if (this.attachedPseudoElements[key]) {
      this.attachedPseudoElements[key].count += 1;
    } else {
      this.attachedPseudoElements[key] = {
        element: element,
        count: 1,
        hydrogenCount: hydrogenCount,
        previousElement: previousElement,
        charge: charge
      };
    }

    this.hasAttachedPseudoElements = true;
  }
Attaches a pseudo element (e.g. Ac) to the atom.
@param {String} element The element identifier (e.g. Br, C, ...).
@param {String} previousElement The element that is part of the main chain (not the terminals that are converted to the pseudo element or concatinated).
@param {Number} [hydrogenCount=0] The number of hydrogens for the element.
@param {Number} [charge=0] The charge for the element.