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Nonterminal symbols are those symbols that can be replaced. They may also be called simply syntactic variables . A formal grammar includes a start symbol , a designated member of the set of nonterminals from which all the strings in the language may be derived by successive applications of the production rules.
It can also be used with highly specific meanings in specialised contexts. In the description of protein structure, in particular in the Protein Data Bank file format, a heteroatom record (HETATM) describes an atom as belonging to a small molecule cofactor rather than being part of a biopolymer chain.
The third gives symbols listed elsewhere in the table that are similar to it in meaning or appearance, or that may be confused with it; The fourth (if present) links to the related article(s) or adds a clarification note.
A terminal symbol, such as a word or a token, is a stand-alone structure in a language being defined. A nonterminal symbol represents a syntactic category, which defines one or more valid phrasal or sentence structure consisted of an n-element subset. Metasymbols provide syntactic information for denotational purposes in a given metasyntax.
Here, the nonterminal T can generate all strings with more a's than b's, the nonterminal U generates all strings with more b's than a's and the nonterminal V generates all strings with an equal number of a's and b's. Omitting the third alternative in the rules for T and U does not restrict the grammar's language.
All the points illustrated by circles and diamonds are nodes in Figure 1, and the former are called nonterminal nodes and the latter terminal nodes. [2]
A context-sensitive grammar is a noncontracting grammar in which all rules are of the form αAβ → αγβ, where A is a nonterminal, and γ is a nonempty string of nonterminal and/or terminal symbols. However, some authors use the term context-sensitive grammar to refer to noncontracting grammars in general. [1]
<symbol> [2] is a nonterminal variable that is always enclosed between the pair <>. ::= means that the symbol on the left must be replaced with the expression on the right. __expression__ consists of one or more sequences of either terminal or nonterminal symbols where each sequence is separated by a vertical bar "|" indicating a choice , the ...