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(bison.info)Using Precedence

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Specifying Operator Precedence
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   Bison allows you to specify these choices with the operator
precedence declarations `%left' and `%right'.  Each such declaration
contains a list of tokens, which are operators whose precedence and
associativity is being declared.  The `%left' declaration makes all
those operators left-associative and the `%right' declaration makes
them right-associative.  A third alternative is `%nonassoc', which
declares that it is a syntax error to find the same operator twice "in a
row".

   The relative precedence of different operators is controlled by the
order in which they are declared.  The first `%left' or `%right'
declaration in the file declares the operators whose precedence is
lowest, the next such declaration declares the operators whose
precedence is a little higher, and so on.