presto-main/src/main/java/io/prestosql/operator/TypeSignatureParser.java/TypeSignatureParser/parseTypeSignatureParameter
class TypeSignatureParser:    private static TypeSignatureParameter parseTypeSignatureParameter(
            String signature,
            int begin,
            int end,
            Set<String> literalCalculationParameters)
    {
        String parameterName = signature.substring(begin, end).trim();
        if (isDigit(signature.charAt(begin))) {
            return TypeSignatureParameter.numericParameter(Long.parseLong(parameterName));
        }
        else if (literalCalculationParameters.contains(parameterName)) {
            return TypeSignatureParameter.typeVariable(parameterName);
        }
        else {
            return typeParameter(parseTypeSignature(parameterName, literalCalculationParameters));
        }
    }