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About 18 months ago, novice entrepreneur Sue Khim flew to San Francisco from her home in Illinois to take part in an uncommonly public version of a Silicon Valley rite of passage — the pitch. With thousands of other young techies in the audience, she was scheduled to be onstage at the Launch Festival, a showcase for “stealth” startups that have managed to keep their products out of the voracious tech press, or have as-yet-unreleased products to announce.