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- In the Chomsky hierarchy of grammars, parse trees can be defined only for context-free grammars . For example, the derivation of 'aaabbbccc' shown in 'Context-sensitive grammar#Examples' doesn't correspond to a tree. (en)
- In the previous paragraph, weak equivalence was introduced as a relation between two grammars. In the current paragraph, it is used a relation between two 'formalisms', i.e. two classes of grammars. (en)
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