Assertion: hang glider's wing | /r/HasProperty | developed by early 1970s
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Source sentences:
text: Although by the early 1970s many rigid wings were developed, none sold terribly well, while dozens of flexible-wing hang glider companies were springing up all over the world.
text: Although by the early 1970s many rigid wings were developed, none sold well, while dozens of flexible-wing hang glider companies were springing up worldwide, building variants of Dickenson's Ski Wing.