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# __construct()
 -  throws an RuntimeException if you don't pass an expected option
# ->clean()
 -  checks that the value is an expected value
 -  checks that the value is an expected value
 -  throws an sfValidatorError if the value is not an expected value
 -  throws a sfValidatorError
# ->asString()
 -  returns a string representation of the validator
# choices as a callable
 -  can take a sfCallable object as a choices option
 -  throws an sfValidatorError if the value is not strictly an expected value
 -  throws a sfValidatorError
 -  throws an sfValidatorError if the minimum number of values are not selected
 -  throws a sfValidatorError
 -  throws an sfValidatorError if more than the maximum number of values are selected
 -  throws a sfValidatorError
# Looks like everything went fine.