This function creates a label.
The identity of the label.
A segment must be open when this call is issued. The position of the next element to be created is used for the label. Subsequently, the method R2::GoToLabel may be used to position to the labelled element. Note that immediately following a call to this method, output to the space should be effected. Calls to R2::SetElementPointer or R2::OffsetElementPointer should not be made in the interim period (that is, prior to the output of an element).