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Two generic types of simple loop geometries (in the circle-line approximation): (a) the subcritical loop with opening angle α < π / 2, and (b) the supercritical loop with α > π / 2. In the former case, the introduction of further excess length leads to an energy increase, but in the latter case, to a relaxation of stress; the introduction of additional length at points XL and XR followed by a relaxation of the structure obviously decreases the total energy.
