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# __construct()
 -  can take no argument
 -  can take a validator as its first argument
 -  can take an array of validators as its first argument
 -  throws an exception when passing a non supported first argument
# ->addValidator()
 -  adds a validator
# ->clean()
 -  returns the string unmodified
 -  throws an sfValidatorError exception if the input value is required
 -  throws a sfValidatorError
 -  throws an sfValidatorError exception if all the validators fails
 -  throws an exception with all error messages
 -  throws a sfValidatorSchemaError
 -  throws a sfValidatorSchemaError
 -  throws an sfValidatorError exception if one of the validators fails
 -  throws a sfValidatorError if invalid message is not empty
 -  throws a sfValidatorError if invalid message is not empty
 -  returns the string unmodified
# ->asString()
 -  returns a string representation of the validator
 -  returns a string representation of the validator
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