code: public function apply(Scope $scope, GenericList $arguments) {
  		if (count($arguments) == 0) throw new \BadFunctionCallException("SymbolExists: No parameters found.");
  		$ref = $arguments[0]->evaluate($scope);
  		if (!is_string($ref) || empty($ref))
  			throw new \InvalidArgumentException("SymbolExists: Symbol must be specified as a non-empty string.");
  		return array_key_exists($ref, $scope->symbols);
  	}
docs: Checks if the given symbol exists on current environment
  Usage: (sym-exists "true") (sym-exists "_value")
  Returns: boolean
  (non-PHPdoc)
  @see \eMacros\Applicable::apply()