<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def check_string(sql_string, add_semicolon=False):
 """ Check whether a string is valid PostgreSQL. Returns a boolean indicating validity and a message from ecpg, which will be an empty string if the input was valid, or a description of the problem otherwise. """

    prepped_sql = sqlprep.prepare_sql(sql_string, add_semicolon=add_semicolon)
    success, msg = ecpg.check_syntax(prepped_sql)
    return success, msg