Expression of a symbol uniquely identified by the template parameter type tag 
| typedef tag | Tag | 
| SymbolValue< Tag > | operator= (Index val) const | 
|  | |
| Index | eval (const T &values) const | 
| typedef tag Eigen::symbolic::SymbolExpr< tag >::Tag | 
Alias to the template parameter tag 
| 
 | inline | 
Associate the value val to the given symbol *this, uniquely identified by its Tag.
The returned object should be passed to ExprBase::eval() to evaluate a given expression with this specified runtime-time value.
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