MultiCurve is defined as a one-dimensional geometry.
A MultiCurve is simple if and only if all of its elements are simple, the only intersections between any two elements occur at points that are on the boundaries of both elements.
The boundary of a MultiCurve is obtained by applying the ``mod 2 union rule'' (also known as the odd-even rule): A point is in the boundary of a MultiCurve if it is in the boundaries of an odd number of MultiCurve elements.
A MultiCurve is closed if all of its elements are closed.
The boundary of a closed MultiCurve is always empty.