This function inserts a LaTeX directive to draw a full or partial line in a table.
Which columns should receive the line?
Hline() is not very flexible: it must be the leftmost header in a row specification
for the table, i.e.
mean * Hline() is not allowed. Anything to the right of the
Hline() factor will be ignored.
Produces an expression to insert a label which will be interpreted by LaTeX as a request for a horizontal line.