Plumber, 244 176 bytes

  []  []      []
      ][=][]
  []=][][=][
[]=]===]]
][]][=[=]]    =]]
[]][   ][===[=[]
 ]][]=][[[=]=]][][
  []]=]=]=]=[]
  ][]=[]][
  []]=][=][]
= =]=]][[=[==
][=]=]=]=]===][=

Old Design (244 bytes)

  []                []
            [][=[]
 [[=[][]  [][]  ][]][=
][[]  ][[=]]]][]=[[]
[]===][]  ][===][]
][][  []    ][  ]     [][]
      [     ][]=][[[===][[
][]]]=][[]=[=]=]=]]==]=]][
  ][=]=]]==]=][]  ][
    [][=][=[[][[=]][
   ==]=]][[=[= =

I thought I had a good solution for a while...except it didn't work with 0 or 1 as the first input. This program consists of three parts: incrementer, decrementer, and outputter.

The decrementer is on the left, and holds the first value. It is incremented, since a 0 would cause an infinite loop (as the program tries to decrement to 0 and gets a negative value). It will decrement the stored value at execution, and send a packet to the incrementer.

The incrementer is on the right, and stores the second input. When the decrementer sends a packet, it increments its stored value, and sends a packet back to the decrementer.

Whenever the decrementer sends the packet to the incrementer, it is checked by the outputter. If the packet is 0, execution stops and the value of the incrementer is read and outputted. It uses a NOT gate, one of the most complicated parts of the program.

Older design (doesn't work with some inputs), 233 bytes

          []          []
          [][]  [][]  =][]
    []=][]=][[=]][
      [][===][  [][===[]
[][]   [        []    ][][
] [===][=]][     ]
][[=[=[=[=[=]=[]][=[[[][
    ][[=[=[=[==[[=[=][
          []]=][=][]
        = =]=]][[=[== 

The new one is flipped to save bytes, and it's really confusing me. This design is the one I worked with for a week or so when designing the interpreter.

Plumber, 244 176 bytes

  []  []      []
      ][=][]
  []=][][=][
[]=]===]]
][]][=[=]]    =]]
[]][   ][===[=[]
 ]][]=][[[=]=]][][
  []]=]=]=]=[]
  ][]=[]][
  []]=][=][]
= =]=]][[=[==
][=]=]=]=]===][=

Old Design (244 bytes)

  []                []
            [][=[]
 [[=[][]  [][]  ][]][=
][[]  ][[=]]]][]=[[]
[]===][]  ][===][]
][][  []    ][  ]     [][]
      [     ][]=][[[===][[
][]]]=][[]=[=]=]=]]==]=]][
  ][=]=]]==]=][]  ][
    [][=][=[[][[=]][
   ==]=]][[=[= =

I thought I had a good solution for a while...except it didn't work with 0 or 1 as the first input. This program consists of three parts: incrementer, decrementer, and outputter.

The decrementer is on the left, and holds the first value. It is incremented, since a 0 would cause an infinite loop (as the program tries to decrement to 0 and gets a negative value). It will decrement the stored value at execution, and send a packet to the incrementer.

The incrementer is on the right, and stores the second input. When the decrementer sends a packet, it increments its stored value, and sends a packet back to the decrementer.

Whenever the decrementer sends the packet to the incrementer, it is checked by the outputter. If the packet is 0, execution stops and the value of the incrementer is read and outputted. It uses a NOT gate, one of the most complicated parts of the program.

Older design (doesn't work with some inputs), 233 bytes

          []          []
          [][]  [][]  =][]
    []=][]=][[=]][
      [][===][  [][===[]
[][]   [        []    ][][
] [===][=]][     ]
][[=[=[=[=[=]=[]][=[[[][
    ][[=[=[=[==[[=[=][
          []]=][=][]
        = =]=]][[=[== 

The new one is flipped to save bytes, and it's really confusing me. This design is the one I worked with for a week or so when designing the interpreter.

Plumber, 244 bytes

  []                []
            [][=[]
 [[=[][]  [][]  ][]][=
][[]  ][[=]]]][]=[[]
[]===][]  ][===][]
][][  []    ][  ]     [][]
      [     ][]=][[[===][[
][]]]=][[]=[=]=]=]]==]=]][
  ][=]=]]==]=][]  ][
    [][=][=[[][[=]][
   ==]=]][[=[= =

I thought I had a good solution for a while...except it didn't work with 0 or 1 as the first input. This program consists of three parts: incrementer, decrementer, and outputter.

The decrementer is on the left, and holds the first value. It is incremented, since a 0 would cause an infinite loop (as the program tries to decrement to 0 and gets a negative value). It will decrement the stored value at execution, and send a packet to the incrementer.

The incrementer is on the right, and stores the second input. When the decrementer sends a packet, it increments its stored value, and sends a packet back to the decrementer.

Whenever the decrementer sends the packet to the incrementer, it is checked by the outputter. If the packet is 0, execution stops and the value of the incrementer is read and outputted. It uses a NOT gate, one of the most complicated parts of the program.

Old design (doesn't work with some inputs), 233 bytes

          []          []
          [][]  [][]  =][]
    []=][]=][[=]][
      [][===][  [][===[]
[][]   [        []    ][][
] [===][=]][     ]
][[=[=[=[=[=]=[]][=[[[][
    ][[=[=[=[==[[=[=][
          []]=][=][]
        = =]=]][[=[== 

The new one is flipped to save bytes, and it's really confusing me. This design is the one I worked with for a week or so when designing the interpreter.

Plumber, 244 176 bytes

  []  []      []
      ][=][]
  []=][][=][
[]=]===]]
][]][=[=]]    =]]
[]][   ][===[=[]
 ]][]=][[[=]=]][][
  []]=]=]=]=[]
  ][]=[]][
  []]=][=][]
= =]=]][[=[==
][=]=]=]=]===][=

Old Design (244 bytes)

  []                []
            [][=[]
 [[=[][]  [][]  ][]][=
][[]  ][[=]]]][]=[[]
[]===][]  ][===][]
][][  []    ][  ]     [][]
      [     ][]=][[[===][[
][]]]=][[]=[=]=]=]]==]=]][
  ][=]=]]==]=][]  ][
    [][=][=[[][[=]][
   ==]=]][[=[= =

I thought I had a good solution for a while...except it didn't work with 0 or 1 as the first input. This program consists of three parts: incrementer, decrementer, and outputter.

The decrementer is on the left, and holds the first value. It is incremented, since a 0 would cause an infinite loop (as the program tries to decrement to 0 and gets a negative value). It will decrement the stored value at execution, and send a packet to the incrementer.

The incrementer is on the right, and stores the second input. When the decrementer sends a packet, it increments its stored value, and sends a packet back to the decrementer.

Whenever the decrementer sends the packet to the incrementer, it is checked by the outputter. If the packet is 0, execution stops and the value of the incrementer is read and outputted. It uses a NOT gate, one of the most complicated parts of the program.

Older design (doesn't work with some inputs), 233 bytes

          []          []
          [][]  [][]  =][]
    []=][]=][[=]][
      [][===][  [][===[]
[][]   [        []    ][][
] [===][=]][     ]
][[=[=[=[=[=]=[]][=[[[][
    ][[=[=[=[==[[=[=][
          []]=][=][]
        = =]=]][[=[== 

The new one is flipped to save bytes, and it's really confusing me. This design is the one I worked with for a week or so when designing the interpreter.