[By Line] command extends an existing curve as a straight.
For the prompt
"Specify around the end point of a curve"
, specify by mouse-click around
the end point of the curve to extend.
For the prompt
"Specify the end point of the line"
, specify the end point of the
extended line.
Pressing B key, the extended line will be doubled by extending the line to both end from the touching point. In this case, the line is generated as a new object without being combined to the original curve.