To copy part of a snippet, we use the mouse to highlight just the lines we want to copy to the other side.

We have done that in the screen shot below, and we see that Beyond Compare 3 has highlighted these newly selected lines in green.  We also see that it has placed a new green arrow in the gutter on the left, with a new bracket extending below it.

We can copy just the 3 lines that we have selected, by clicking on the little green arrow in the left gutter.  After we click the green arrow...

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