* @param desiredValue the market data requirement, not null
* @return The satisfaction of the requirement from the underlying providers.
*/
@Override
public ValueSpecification getAvailability(final ComputationTargetSpecification targetSpec, final Object target, final ValueRequirement desiredValue) {
MarketDataNotSatisfiableException missing = null;
for (int i = 0; i < _providers.size(); i++) {
final MarketDataAvailabilityProvider provider = _providers.get(i);
try {
final ValueSpecification underlying = provider.getAvailability(targetSpec, target, desiredValue);
if (underlying != null) {