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ST construction in tree grammars
Tree grammars match subtrees constructed by a parser. In order to create an output template using a tree grammar, the tree grammar must know about the token stream from which its trees were created. If you rewrite the tree, all of the token indexes will be incorrect. If a node for ID was originally created from a token at index 32, but you move it around in the tree, this pretty much preventing ANTLR from creating a valid string derived from the input. So, Automatic construction of templates only works if you have not manipulated the tree.
Each tree grammar rule knows the text from which the associated subtree was created but only if the subtree has a single root. The following rule, because it has a single root, gives ANTLR a problem.
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decls : decl+ ;
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ANTLR is not currently automatically figure out the last sibling, which means that it cannot figure out the complete text automatically for an arbitrary rule; so, stick to rules with a single root node. Fortunately, that is the common case; e.g.,
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decl : ^(DECL type ID) ;
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