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# __construct()
 -  throws an RuntimeException if you don't pass a callback option
# ->configure()
 -  switch required to false by default
# ->clean()
 -  calls our validator callback
 -  throws a sfValidatorError
 -  throws a sfValidatorError
# callback with arguments
 -  can take an arguments option
# ->asString()
 -  returns a string representation of the validator
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