code: func (statement *Statement) Decr(column string, arg ...interface{}) *Statement {
  	k := strings.ToLower(column)
  	if len(arg) > 0 {
  		statement.decrColumns[k] = decrParam{column, arg[0]}
  	} else {
  		statement.decrColumns[k] = decrParam{column, 1}
  	}
  	return statement
  }
docs: // Decr Generate  "Update ... Set column = column - arg" statement