How to count the total lake area in an Atlas

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Once I have loaded an Atlas object in memory, what is the best way to count the total lake area? Doing a simple tag search finds all the simple Area lakes but I am missing all the lakes based on type=multipolygon Relations that are built on "outer" ways that, stitched together, make a full lake.

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Atlas comes with a ComplexEntity concept that allows the user to construct higher-level entities based on specific concepts. The ComplexWaterEntity object should fit that need:

Atlas atlas;
Iterable<ComplexWaterEntity> waterEntities =
    new ComplexWaterEntityFinder().find(atlas);

// Get all water bodies and keep lakes only.
// This will include the multipolygon ones.
Iterable<ComplexWaterBody> lakes =
    .filter(entity -> WaterType.LAKE == entity.getWaterType())
    .map(entity -> (ComplexWaterBody) entity);

// Add all the surface areas
Surface result = Surface.MINIMUM;
for (ComplexWaterBody lake : lakes)
    result = result.add(lake.getGeometry().getSurface()));

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