code: func Contains(a interface{}, b interface{}) BoolPredicate {
  	return func() bool {
  		aval, ok := a.([]string)
  		if !ok {
  			return false
  		}
  		bval, ok := b.(string)
  		if !ok {
  			return false
  		}
  		for _, v := range aval {
  			if v == bval {
  				return true
  			}
  		}
  		return false
  	}
  }
docs: // Contains checks if string slice contains a string
  // Contains([]string{"a", "b"}, "b") -> true