def delete_all(pattern = {})
      result = true
      each(pattern) { |st| result &&= delete(st) }
      result
    end
def delete_all(pattern = {})
      result = true
      each(pattern) { |st| result &&= delete(st) }
      result
    end
Delete all statements from the model,
 matching the given pattern

 @param [Statement, Hash] pattern (see {#find})
 @return [Boolean]