Dispatch
 Usage
• Dispatch[{ -> ,  -> , ... }] generates an optimized dispatch table representation of a list of rules. The object produced by Dispatch can be used to give the rules in expr /. rules.
Notes
• The use of Dispatch will never affect results that are obtained, but may make the application of long lists of rules much faster. • Lists of rules are usually scanned sequentially when you evaluate an expression like expr /. rules. Rules such as a[1]->a1 and a[2]->a2, which cannot simultaneously apply, need not both be scanned explicitly. Dispatch generates a dispatch table which uses hash codes to specify which sets of rules need actually be scanned for a particular input expression. • Lists of rules produced by assignments made with = and := are automatically optimized with dispatch tables when appropriate. • New in Version 2.
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