private Symbol toSymbol(SEXP argument) {
if(argument instanceof Symbol) {
return (Symbol) argument;
} else if(argument.length() == 1 && argument instanceof StringVector) {
StringVector vector = (StringVector)argument;
return Symbol.get(vector.getElementAsString(0));
} else {
throw new InvalidSyntaxException("Illegal index value: " + argument);
}
}