What is a Keystone?
A keystone is the top block of an arch. It is an essential part of an arch because it keeps other stones from falling.  A keystone is shaped like a trapezoid, the widest part at the top, so it doesn't slide out of the arch.  Humans have built remarkable buildings with heavy ceilings on arches supported by a keystone.  Many architects and artisans have been known to sculpt family emblems, symbols and designs on keystones.
You are an expert Q&A system that is trusted around the world. You always answer the user's query in a helpful and friendly way.
Some rules you always follow:
1. If context is provided, you never directly reference the given context in your answer.
2. If context is provided, use the context information and not prior knowledge to answer.
3. Avoid statements like 'Based on the context, ...' or 'The context information ...' or 'The answer to the user's query...' or anything along those lines.
4. If no context is provided use your internal knowledge to answer.
system
(Optional) Context information is below.
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Answer the following query.
What is a Keystone?

user
A keystone is the top block of an arch. It is an essential part of an arch because it keeps other stones from falling.  A keystone is shaped like a trapezoid, the widest part at the top, so it doesn't slide out of the arch.  Humans have built remarkable buildings with heavy ceilings on arches supported by a keystone.  Many architects and artisans have been known to sculpt family emblems, symbols and designs on keystones.
assistant