Examples of the principle of annulling a gap if the surrounding sections of wall are longer than the gap [Omeid Merubeh Al haParutz]: Eruvin 5a, 5b, 10a-b, 15b-16b
Trying to combine two surrounding sections of wall, to cancel out the intervening gap: Eruvin 10b [2x], 16b
What happens if a gap is equal to the section of wall around it [Omeid kiParutz]: Eruvin 10b [2x], 12b, 15b-16b; Succah 15a
Differentiating between vertical and horizontal gaps: Eruvin 16b