The buttons such as
Set all lines,
Set right line,
Set left line and
Set bottom line are not activated with arrays as the example on the left while they are activated on the example on the right with tabular environment.
My goal is to learn to create upper triangularizations like on the right below. For that one needs to turn the array into tabular environment somehow or find a way to get the line options in matrices/arrays/tabulars activated.
How can I add the tabular lines or matrix lines in LyX?