表达式28: 0 less than or equal to "y" less than or equal to "l" left brace, "b" minus 2 less than or equal to "x" less than or equal to "b" , right brace0≤y≤lb−2≤x≤b
28
表达式29: "b" minus 2 less than or equal to "x" less than or equal to "b" left brace, 0 less than or equal to "y" less than or equal to "l" , right braceb−2≤x≤b0≤y≤l
