BNFNode :: symbol - Get symbol name for BNF tree node
The function returns the symbol name for the BNF tree node, e.r. the name defined in the BNF definition as symbol name (left side). When the BNF tree node is not valid, the function throws an exception.
Return value: BNF symbol name ( odaba::String & )
The BNF symbol name is a name that has been defined as symbol in the BNF parser definition.