How to compose left-right symmetrical shapes?

[Contours]

I've used Xara for a couple of years ... but I've never found an easy way to build symmetrical shapes (so that the left half exactly matches the right half). I'm not talking about circles or elipses or such, but shapes that require adjustments to the curves involved.

Typically, I start such a shape by clicking four, six or eight points together to form the basic shape ... and then set about modifying the curves between the points to get the shape. But it's then a tedious process to get one side to match the other.

Using the simple shape below as an example, what's the shortest way to get the result? The only other way I've tried is bisecting a half-built shape ... duplicating ... flipping the duplicate ... and then joining the halves. But that procedure has some problems of its own.

Thanks for any help.