This editor is used to edit the selected formula.
The value in this combo-box specifies the current library mode, which determines
what items are shown in the Library text box.
The items shown in this text box are elements that can be inserted into
the formula. Left double clicking an item inserts it into the formula in the Formula text box.
This text box displays the description for the item selected in the Library
text box. This text box is read only.
This text box displays the formula expression. The user may directly edit the
formula in this text box.