public PredictionContext get(PredictionContext ctx) {
		return cache.get(ctx);
	}
Add a context to the cache and return it. If the context already exists,
 return that one instead and do not add a new context to the cache.
 Protect shared cache from unsafe thread access.
runtime/Java/src/org/antlr/v4/runtime/atn/PredictionContextCache.java