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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def execution_context_from_async(async):
 """Instantiate a new _ExecutionContext and store a reference to it in the global async context to make later retrieval easier. """

    local_context = _local.get_local_context()

    if local_context._executing_async_context:
        raise errors.ContextExistsError

    execution_context = _ExecutionContext(async)
    local_context._executing_async_context = execution_context
    return execution_context