Return a list of attribute references in the condition expression.

    attribute_reference ::= relation_name "." attribute_name | attribute_name

    :param instring: a condition expression.
    :return: a list of attribute references.

def get_attribute_references(instring):
    """
    Return a list of attribute references in the condition expression.

    attribute_reference ::= relation_name "." attribute_name | attribute_name

    :param instring: a condition expression.
    :return: a list of attribute references.
    """
    parsed = ConditionGrammar().conditions.parseString(instring)
    result = parsed if isinstance(parsed[0], str) else parsed[0]
    return result.attribute_reference.asList() if result.attribute_reference else []