code: public function apply(Scope $scope, GenericList $arguments) {
  		if (count($arguments) == 0) throw new \BadFunctionCallException("SymbolSet: No parameters found.");
  		$ref = $arguments[0]->evaluate($scope);
  		if (!is_string($ref) || empty($ref)) throw new \InvalidArgumentException("SymbolSet: Symbol must be specified as a non-empty string.");
  		Symbol::validateSymbol($ref);
  		$value = (count($arguments) > 1) ? $arguments[1]->evaluate($scope) : null;
  		$scope->symbols[$ref] = $value;
  	}
docs: Defines a symbol value
  Usage: (sym "value") (sym "counter" 0)
  Returns: NULL
  (non-PHPdoc)
  @see \eMacros\Applicable::apply()