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