The Bevel button .
If the segments intersect - superfluous parts are automatically cut off after the use of this tool. If the line segments do not intersect, the segments can be extended. The bevel button 
can be found in the Editor tab under the Instruments section.
Select the first line segment and press the Bevel button (Editor Tab -> Instruments -> Bevel). Select the second line segment.
Example 1: intersecting lines
before: 
after: 
Example 2: non intersecting lines
before: 
after: 
Note: bevel's value is available for changing:
| • | select the first line segment; |
| • | press Bevel button; |
| • | place mouse cursor over the drawing field; the field in the dynamic input shows the current bevel value  (physically this is the distance from the intersection point to the sections' cut off points) |
| • | delete the current value using Backspace key and set the new one; |
| • | press Enter key; |
| • | select the second line segment. |