<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _expand_placeholder_value(value):
 """ Return the SQL string representation of the specified placeholder's value. @param value: the value of a placeholder such as a simple element, a list, or a tuple of one string. @note: by convention, a tuple of one string indicates that this string MUST not be quoted as it represents, for instance, a called to a stored procedure, and not a textual content to modify into a table. @return: a SQL string representation. """

        if isinstance(value, (list, set)) or (isinstance(value, tuple) and len(value) != 1):
            sql_value = ','.join( [ RdbmsConnection._to_sql_value(
                        element if not isinstance(element, tuple) else element[0],
                        noquote=isinstance(element, tuple))
                    for element in value ])

        elif isinstance(value, tuple):
            assert len(value) == 1
            value = value[0]
            assert value is None or isinstance(value, basestring), 'basestring expected instead of %s' % type(value)
            sql_value = RdbmsConnection._to_sql_value(value, True)

        else:
            sql_value = RdbmsConnection._to_sql_value(value)

        return sql_value