Typo Tracker

The last table in Sec. 5.5 does not appear to correspond to the code before it. Notice that the code has Sec. 5.5 does not appear to correspond to the code before it. Notice that the code has \emph{Missing} but the displayed table has blanks

NLCT: This is not a typo. The aim of the example is to show the difference between null and empty values. The table has empty not null values. I will modify the text in the next version to clarify this.